Because of the current demographic trends more and more older people need adequate support in their daily activities. An increasingly important role here are getting the information and communication technologies and services that enhance the quality of life of elderly people and extend their independence. The term Ambient Assisted Living (AAL) entered the public with the EU Framework Programme support action aimed towards providing support to the elderly people in their homes, on the move, in society and at work. A number of research initiatives and multidisciplinary programs have been established around the field of development for ageing population in the last years. Although they operate in a very challenging environment and have to deal with a vast number of ever-changing requirements they strive to bring advances that will help this very specific end user group. The challenges in the area of AAL can be viewed from the standpoint of end user(s), developers, providers, industry, governments and other relevant stakeholders and include issues related to technology acceptance, trust, heterogeneity, interoperability, standardisation, security, privacy, legal aspects, reliability, undefined market, etc. In order to facilitate the introduction of new software systems and applications in the field of assisted living the services should be easy to use for end users and easy to develop for developers. Older people and people with disabilities have the same need for accessing information and different applications as everyone else but require a different approach. Designing user interfaces for elderly and/or disabled is a big challenge since they have various and special needs that have to be considered in every aspect of a design process. It is not unusual for older people to experience restricted mobility, declining vision, hearing and reaction time which means that they can can greatly benefit from new and more appropriate ways of interaction that would lower the interaction barrier. User interfaces that tailor interaction and presentation capabilities based on user capabilities, preferences and expectations can significantly increase the usability of systems in the area of AAL. Furthermore, context-aware systems using information about the interaction environment, end user(s) and the system itself can bring even more proactivity and adaptivity to interaction between system and its end user(s). There was a major shift in the way technology is used today. Many advances in science and technology enable the development of services that were previously not possible. New services offer a growing set of functionalities that they want to deliver to end users. This creates additional pressure on developing clear and intuitive user interfaces, especially when it comes to elderly people with specific needs, capabilities and preferences. The vast majority of existing interfaces does not adequately fulfill accessibility requirements of elderly users suffering from visual, hearing or any other impairment. For this reason, demand for multimodal user interfaces that allows the use of different communication channels for communication with users and systems will surely grow in the future. The mechanism of selection and adaptation of user interfaces for multimodal interaction based on a comparison of the product's capabilities and features and user preferences can improve customer satisfaction and performance compared to traditional user interfaces. The dissertation gives basic information about the graying society and evolution of user interaction in this area. The research was conducted from the technological and sociological aspects in order to identify major conceptual characteristics and specific requirements for developers in this area. Systems in the area of AAL are focused towards enabling independence, autonomy, security, and generally necessary assistance and quality of life to the elderly. For this reason, they need appropriate user interfaces that can meet the demands of this, in many ways a very specific and heterogeneous user group. In order to help the users to take advantage of the full potential of communication with such systems new technological solutions that enable greater flexibility in selecting the most appropriate modalities of interaction and better customization of user interfaces according to the capabilities and preferences of end-users are of a great importance. The dissertation gives insights into intelligent user interfaces, describes basic concepts related to multimodal user interfaces and most important models of interactive systems. It identifies most common approaches used in the development of user interfaces for the elderly, different types of adaptation of user interfaces and existing standards and guidelines for the development of user interfaces intended mainly for the elderly people. Based on proposed criteria different user interaction frameworks in AAL systems were analyzed. The performed analysis was focused towards understanding: i) how the input was gathered from users and how the AAL system presented output, as well as which devices and/or modalities were used during the interaction process, ii) whether pluggable user interfaces, allowing easy change of user interfaces without (or with minimal) changes in the system application layer, were used, iii) whether modality independent user interface model was used and, finally, special focus was put on iv) support for adaptation in terms of identifying who initiated the adaptation (user, system or both) and what was the object of the adaptation (structure, presentation and/or content). Since considered projects producing different platforms had different scopes and goals, their architectures and user interaction frameworks were different but always focused towards responding to end user needs as much as possible. From the survey results it is obvious that more traditional ways of output (such as graphical user interfaces, keyboards and computer mouses) are used in most cases but also that new ways of interaction are starting to be more and more popular. Touch screens as well as voice and gesture interaction are especially being recognized to be suitable for input for this target group. Results of the performed analysis of user interaction in existing AAL platforms lead to the conclusion that there are many projects but not yet common platform(s) that can really be considered the standard. In most user interaction frameworks certain common characteristics can be identified, but still a lot of work is needed in order to identify the best approach. In order to be capable of smart responses, user interfaces will have to know their users and their environment and be able to adapt. In order to achieve good user interface adaptation underlying user models and contextual awareness are essential. Expansion of user modelling field in recent years has also in part been triggered due to the growing amount of data that can be collected from users and their environments. Availability of this new data has, in turn, created new opportunities in the field of user interaction. The dissertation proposes semantic description of interaction capabilities and preferences of users based on the existing standards. User interaction capabilities are divided into cognitive, sensory and psychomotor while user interaction preferences are divided into general, visual, auditory, olfactory, gustatory and tactile preferences. Each group is then explained in more details. For example, general user interaction preferences relate to general information about the (non)preferred modes of interaction, preferred language(s) and time of interaction, currency, date format, method of delivery and presentation of alarms and notifications and the manner and conditions under which the system should initiate the interaction. The more complex rules defined within the general interaction preferences may contain conditions that take into account the context of use and other aspects such as the affect, the emotional state of the user, etc. The proposed semantic description of the interaction capabilities and preferences of users is used in order to optimize the mechanism of selection and adaptation of user interfaces within the framework of multi-modal user interaction that supports the independence of the application and presentation layer and which further allows the inclusion of contextual data. The proposed semantic description of the interaction capabilities and preferences of users is implemented and integrated into a user interaction framework of the reference AAL platform universAAL. For the representation of the semantic description of interaction capabilities and preferences of users ontologies were used. The universAAL Framework for User Interaction in Multimedia, Ambient Assisted Living (AAL) Spaces was accepted as Publicly Available Specification by the International Electrotechnical Commission under the reference IEC/PAS 62883 Ed. 1.0 in March 2014. EU-FP7 project universAAL had the goal to design and develop an open platform that provides a standardized approach for an easy and economic development of AAL solutions. The universAAL platform is intended to benefit end users (i.e., assisted persons, their families, and caregivers), authorities with responsibility for AAL, and organizations involved in the development and deployment of AAL services. Some results described in this dissertation were incorporated in the universAAL user interaction framework which components and most important concepts were described here. The research hypothesis that semantic web technologies and semantic description of interaction capabilities and preferences of users can be used to optimize the mechanism of selection and adaptation of user interfaces for multimodal interaction in the AAL system was confirmed. The dissertation showed the possibility of replacing existing, traditional ways of designing the application for which each application produces a specific user interface with the new way which is based on the separation of the application and presentation layers and which includes a special framework for the user interaction, which adapts the interaction taking into account the information about the context alongside the capabilities and preferences of the user. For the separation of application and presentation logic abstract user interfaces that are modality and device agnostic are used. By including platform specific information abstract user interfaces are transformed into concrete user interfaces. At runtime, when the actual context of use becomes known, the final user interface is built as an instantiation of the abstract user interface. In universAAL user interaction framework the XForms based abstract user interfaces were used. The format of the abstract user interface can come in many flavours as is also visible from the overview of the User Interface Description Languages given in the disseration. Some approaches employ task models, some use abstract forms of widgets and controls, some user interface parametrization, etc. With an advancement of technology user interaction has also evolved and will continue to do so. Our activities are increasingly becoming mediated by different devices such as computers or mobile phones. Many solutions that are being developed are rather complex what results in the fact that more attention has to be directed towards designing usable user interfaces. Many advances in science and technology allow developing services that were formerly not possible. User interface adaptation is a challenge to accomplish and with the advances in Ambient Intelligence difficulties are constantly growing which means that more research is needed in this exciting field. In mixed initiative interaction control over the adaptation process is divided between the user and the system. In further research, it would be good to explore how much control users are willing to leave to the system and whether, and if so how, their attitude changes in relation to their demographic age. The development of technology leads to the emergence of new devices that can be used to improve the process of interaction. It would be good to investigate the way in which these new devices and solutions affect the lives of people with different personal and social norms.