The Glutathione S-transferases (GSTs) form a group of multi-gene isoenzymes involved in the cellular detoxification of both xenobiotic and endobiotic compounds. GSTs have been divided into a number of subclasses, alpha, mu, pi, and theta. The classification was made on the basis of sequence similarity and immunological cross-reactivity. GSTs show a high level of specificity toward GSH but the electrophilic second substrate can vary significantly both between and within the classes in spite of their sequence similarity. X-ray crystallography and site-directed mutagenesis studies have together elucidated the structure and mechanism of GSTs. Catalysis occurs by conjugation with glutathione (GSH) and the less toxic and more hydrophilic products can then be partially metabolised and excreted. This invaluable service is however disadvantageous during chemotherapy where GSTs have been associated with multi-drug resistance of tumour cells. Levels of expression of different isoforms of GSTs are tissue specific. The variations in expression between normal and tumour cells are of interest and in most cases the levels of GSTs are increased, especially p-GST. Understanding the complex role that GSTs play in drug resistance begins with determining the pattern of isoform expression and the substrate specificities of each isoform. The use of isozyme-specific, GSH analogues as inhibitors to modulate GST activity during chemotherapy is a promising strategy in the battle against cancer. This review attempts to provide a detailed overview of the literature concerning the different classes of GSTs, their function and mechanism and the use of GSTs as therapeutic targets for disease as current at the time of submission.