Signal transducer and activator of transcription 3 (STAT3) is a latent 
cytoplasmic transcription factor, originally discovered as a transducer of 
signal from cell surface receptors to the nucleus. It is activated by tyrosine 
phosphorylation at position 705 leading to its dimerization, nuclear 
translocation, DNA binding, and activation of gene transcription. Under normal 
physiological conditions, STAT3 activation is tightly regulated. However, 
compelling evidence suggests that STAT3 is constitutively activated in many 
cancers and plays a pivotal role in tumor growth and metastasis. It regulates 
cellular proliferation, invasion, migration, and angiogenesis that are critical 
for cancer metastasis. In this paper, we first describe the mechanism of STAT3 
regulation followed by how STAT3 is involved in cancer metastasis, then we 
summarize the various small molecule inhibitors that inhibit STAT3 signaling.