A virtual or mixed reality environment for neurological rehabilitation should simulate the rehabilitation of the task, and not simply simulate the task. This involves identifying the errors in task performance that neurologically damaged patients make during task performance and replicating the guidance given during skilled rehabilitation. Involvement of a skilled therapist in the design and development team is essential. Neurological rehabilitation is complex and replicating it requires compromises between the desire to replicate this complex process and the amount of development time. Virtual or mixed reality systems that can simulate the rehabilitation process are suitable for clinical effectiveness studies.