This thesis evaluates the Ultra High Risk (UHR) for Psychosis evidence, and seeks to clarify how research exploring differences demonstrated by cannabis and ketamine users on measures and tasks related to the psychosis prodrome can contribute to understanding the factors involved in this stage. The thesis also examines the literature on cognitive biases and insight in the UHR state, summarising the evidence for measures and tasks which sensitively differentiate the UHR state from other stages of Psychosis development.