During the past decade, genomic analyses have been introduced into cancer studies with variable success. It has become recognised, however, that genomic techniques in isolation are insufficient to study the complex pathways of carcinogenesis; this has led to the application of proteomic techniques, which allow for the reliable analysis of complex mixtures of proteins. This article reviews the basic principles of proteomics, methods currently used in proteomics including two-dimensional gel electrophoresis (2-DE) and mass spectrometry (MS), and the application of proteomics in cancer research. Currently, proteomic technology has been used in two main areas of cancer research: early diagnosis and treatment (included prediction of response to treatment and targeting novel cancer agents). The initial results from both in vitro and in vivo studies are impressive. These technologies, particularly when combined with genomic analyses, will provide valuable insights into the molecular basis of carcinogenesis and the development of more effective anti-cancer therapies.