From their earliest reports, Kanner and Asperger included the hierarchy of difficulties in socialisation as one of the key axes in persons affected with autism spectrum disorders (ASD), associated to development delay or language disorders and restricted interests. This deficiency in social cognition has been related with a deficit in empathy. The theory of deficit in empathising and hypersystematisation provides a coherent, comprehensible explanation with which to partially understand the genesis of these disorders. Empathy is an essential component for emotional experiencing and social interaction, and denotes an affective response to mental states that are either perceived directly or imagined or are feelings inferred by another person. It enables us to understand, feel and respond appropriately to social stimuli, thereby giving rise to an adequate socialisation. Empathy has been considered a synonym of emotional contagion, mimicry, sympathy, compassion and empathic interest. Although these are concepts that are related and necessary for the development of adequate social cognition, they are not the same; nonetheless, they are all essential for the development of empathy or its consequences. Empathy allows individuals to 'feel with', whereas sympathy, compassion and empathic interest are related with 'feeling for' or feeling what is appropriate. Studies conducted in persons with ASD have shown them to have a low empathy quotient. In this work, different aspects of empathy, its components, its neurobiological foundations, the manifestations related with its deficit and its relation with the development of ASDs are all analysed.