This chapter delineates the most salient factors in the outcome of tuberous sclerosis. It begins with a brief historical overview of the tuberous sclerosis complex (TSC) and proceeds to review current knowledge about the genotype, the physical phenotype and the behavioural phenotype. The chapter highlights the lifespan issues relating to outcome, before summarizing the key issues in assessment and intervention for children and adults with the TSC. The mechanism proposed to account for the variability of expression in tuberous sclerosis is based on Knudson's two hit model of tumour pathogenesis. Tuberous sclerosis can give rise to lesions virtually in any part of the body. The diagnostic criteria highlights the spectrum of features in the skin, eyes, heart, lungs and the Central nervous system (CNS). Brain size is normal in most cases of tuberous sclerosis. The TSC is associated with a spectrum of behavioural, psychological and psychiatric problems.