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Publisher Summary In recent times, tremendous strides have been made toward the identification of the intracellular components involved in signal transduction. The emerging theme from these studies is that the cellular responses are regulated by the activity of multiple parallel signaling pathways. Furthermore, qualitative and quantitative differences in the activities of specific pathways may be critical in defining the cellular response. Protein tyrosine kinases (TK) play a critical role in mediating the cellular responses to the environmental signals. TKs are known to affect a broad range of cellular processes, including proliferation, differentiation, migration, metabolism, and cell death. Alteration in the activity of TKs can lead to profound biological consequences and are associated with a number of human diseases, including cancer, diabetes, and immune dysfunctions. Activated protein tyrosine kinases have been implicated in a broad range of human cancers. One of the best characterized examples of a tyrosine kinase, playing a causative role in human malignancy, is that of the chimeric bcr/abl oncogene in human chronic myelogenous leukemia. This chapter discusses the current understanding of the intracellular signaling pathways activated by protein tyrosine kinases. Particular emphasis is given to the mitogenic pathway and its role in cancer.