Abstract: Intracellular signal transduction plays an important role in the process of cellular metabolism, segmentation, differentiation, biological behaviour and cell death. Overactive signal transduction relates to tumor development and progression. Signaling pathways operated by protein tyrosine kinases (PTKs) will be illuminated here briefly. The Ras/Raf/MAPK and PI-3K/Akt pathways through receptor protein tyrosine kinases (RTKs), the Src, Bcr-Abl and JAK/STAT pathways by non-receptor protein tyrosine kinases (nrPTKs) are shown separately. Antitumor agents targeting the key proteins involved in the above five signalling routes are also summarized in this review.