This chapter first presents the molecular physiology and pathology of tuberous sclerosis complex (TSC), and then discusses diagnostic testing for TSC. In TSC, inadequate suppression of the mTORC1 pathway results in dysgenic lesions and tumor growth in multiple organ systems. Epilepsy is the most common neurological symptom, affecting approximately 85% of TSC patients, and is a significant source of morbidity and mortality. Infantile spasms in TSC frequently develop concurrently with partial epilepsy and poor control of seizures types other than spasms is also correlated with poor outcome. Initial evaluation of a patient with known or suspected TSC should focus on confirmation of the diagnosis by identifying major and minor diagnostic features. Confident diagnosis of TSC in a young child can be difficult, as different organs are preferentially involved at distinct developmental stages. Multicenter clinical trials evaluating rapamycin and other structurally similar chemical analogs are currently underway for use in TSC.