This doctoral dissertation investigates user interface (UI) design, implementation and evaluation for interactive television (ITV) applications. Advances in set-top box technology made possible the digital video recorder (DVR) and Internet connectivity, thus making the television interactive. This work re-examines the established human-computer interaction (HCI) theory against the requirements of ITV applications. 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 software developers and the TV producers, and more crucially, it did not consider the ITV user as a TV viewer. Therefore, we present a conceptual model, a set of user interface principles and an evaluation methodology that are suitable for ITV applications. In addition, we provide the respective user interface programming toolkit.