Human cancer results from genetic and epigenetic changes that affect key cellular processes. Most of these processes, including excessive cell division and attraction of blood vessels to growing tumors, are normally regulated by binding of hormones called growth factors and angiogenic factors to receptors at the cell surface. Activation of the receptors by the hormones, and the resulting cascade of changes inside the cells that determine whether the cells will divide or commit suicide, and also whether growing tumors are likely to invade other tissues and disseminate to form metastases, is called signal transduction. Not surprisingly, the majority of mutations and chromosomal changes that contribute to cancer development affect these same pathways and processes. Thus, elucidation of signal transduction is vital for understanding how cancer arises in each individual, and the features that distinguish benign from aggressive cancers. Most importantly, components of the cellular machinery that mediates receptor-regulated growth processes are among the best rational cancer therapeutic targets in cancer.