Both intellectual disabilities and psychosis are complex conditions affecting cognitive, adaptive, and emotional functioning. Moreover, psychosis is associated with lower cognitive functioning compared to the general population, particularly in those with early onset or very early onset. Adaptive skills are intertwined with cognitive abilities. For persons with both intellectual disabilities and psychosis, adaptive skills may be severely impaired during high-symptom phases. Impairments of emotional development is not one of the main criteria of intellectual disabilities. However, emotional development is delayed and impaired in this group. This chapter describes why emotional development is an important area of knowledge. Emotional development is strongly related to behavioural expressions. Hence, understanding the level of emotional development is crucial for understanding challenging behaviour in people with intellectual disabilities who develop psychosis. A tool for assessing emotional development will be presented.