Abstract. This doctoral dissertation aims to investigate user interface (UI) design, implementation and evaluation for interactive television (ITV) applications. Computer mediated entertainment (e.g. video-games, digital music, DVD movies, ITV) has emerged as a major economic factor in the media industry, taking-up a large portion of consumer spending and leisure time. Advances in set-top box technology made possible the digital video recorder (DVR) and Internet connectivity, thus making the television interactive. The objective of the present work is to evaluate the established human-computer interaction (HCI) theory against the requirements of ITV applications. Previous HCI research about ITV focused only on the design of the electronic program guide (EPG) and did not consider the enhancement of the TV content. Furthermore, previous research approached ITV from a single perspective (e.g. computer engineering, advertising, communication) and it did not consider the conflicts of interest between the broadcasters and the consumers, between the developers and the producers, and more crucially, it did not consider the ITV user as a TV viewer. For this purpose, the established TV watching behavior is identified in other scientific disciplines, such as advertising and communication, and it is combined with the Information Technology (IT) usability mentality. The design methodology involves two phases. The objective of the first design phase is to formulate a small set of principal elements that are generic to the design of ITV UIs, such as Virtual Channel conceptual model, UI principles, UI development toolkit, prototyping platform, and affective usability evaluation framework. The objective of the second design phase is to employ the elements identified in the previous stage into the development of an ITV application. The ITV application was evaluated by consumers and addressed three contemporary UI issues: video skipping, animated character, and dynamic advertisement insertion. Overall, the methodology employed a holistic design approach for ITV applications, in which the UI model and the business model were systematically mapped to and validated through an ITV music application. In brief, it was found that the track-skipping UI seamlessly enhanced consumer entertainment. Moreover, it was found that an animated character is preferred, when compared with the traditional transparent box for the presentation of related information. Consumers evaluated the dynamic advertisement insertion positively, thus, opening-up many opportunities for novel advertising formats. The employment of the proposed HCI elements made the design and the development of the ITV application a straightforward process and produced an entertainment experience that was liked by the consumers. The results entail significant implications for the TV industry. ITV is currently perceived as a set of decorative elements, which do not provide any actual improvement of the existing TV content. On the other hand, video-skipping is a familiar functionality that should be exploited, instead of neglected due to the fear of cannibalizing the fixed advertising of the broadcast schedules. The dynamic advertisement insertion in the TV content stream offers a novel advertising format. Finally, there are opportunities for a new mediating role in the media industry that combines the available broadcast transmission with additional elements (Internet resources, computer generated graphics) for the provision of personalized virtual television channels.