Retinoblastoma is the most common primary intraocular tumor in childhood. Mutations in both the alleles of the RB1 gene represent the causative agent for the tumor to occur. It is becoming evident that, although these alterations represent key events in the genesis of retinoblastoma, they are not sufficient per se for the tumor to develop, and other additional genetic or epigenetic alterations must occur. A supportive role in the genesis of retinoblastoma has recently been proposed for the RB1-related gene RB2/p130. Additionally, several other genetic alterations involving different chromosomes have been described as relevant in the tumorigenic process. In this review we will analyse current knowledge about the molecular mechanisms involved in retinoblastoma, paying particular attention to the mechanisms of inactivation of the biological function of the retinoblastoma family of proteins.