The Id (Inhibitor of differentiation or Inhibitor of DNA-binding) proteins act as dominant negative inhibitors of differentiation-specific basic Helix-Loop-Helix (bHLH) transcription factors. Id proteins negatively regulate cellular differentiation and they induce proliferation by modulating different cell cycle regulators both by direct and indirect mechanisms. Ectopic expression of Id proteins in tissue culture models can result in cellular immortalization and abrogation of differentiation processes. Recent reports show that Id proteins are overexpressed in various cancer types implying a role of these regulatory proteins in carcinogenesis. This review focuses on the biology of the Id proteins and their role as potential oncogenes.