This thesis studies the creation of illusion in computer aided performance. Illusion is created here by using deceptions, and a design framework is presented which suggests several different deception strategies which may be useful. The framework has been developed in an iterative process in tandem with the development of 3 real world performances which were used to explore deception strategies. The first case study presents a system for augmenting juggling performance. The techniques that were developed to control this system demonstrate how deception may become useful even when the core of the performance is not deceptive in any way. This is followed by a magic performance called the Cup Game, which was designed to explicitly test the strategies of deception described in the framework. The final case study is an interactive art installation which presents the illusion of a pet rock that lives in a cage. This demonstrates the usefulness of suspension of disbelief in the creation of illusions. It also demonstrates interesting social effects that are used to strengthen this suspension of disbelief. The idea of creating the impression of a false situation is inspired particularly by previous HCI work on public interaction. This work demonstrated the usefulness of hiding interface use or computer outputs from some people in a situation. The creation of deliberately ambiguous computer interfaces, which allow for a wider variety of interpretations to be made by the user has also been described. The work here goes beyond these techniques to use technology to actively create false impressions. The techniques used in this process are guided by the work of magic performers, and by psychological studies of how magic performance works. As well as artistic performance, it is envisaged that this work may prove applicable to more traditional situations. In addition to the framework itself, the development of the case studies has created several useful algorithms which have wider applications. The case studies are also useful guides for those creating performance systems, or other systems where deceptive techniques may be useful.