Catalase (CAT) is a well-studied enzyme that plays an important role in protecting cells against the toxic effects of hydrogen peroxide. In human, it has been implicated in different physiological and pathological conditions. This review summarizes the information available on the function and role of CAT polymorphisms in pathogenesis of various pathophysiological states as well as on the regulation of CAT gene expression. Numerous studies have described the CAT polymorphisms and their link with various diseases. Changes in the CAT levels were reported in many different diseases and polymorphisms in the CAT gene were shown to be associated with different pathophysiological states, e.g. hypertension, diabetes mellitus, insulin resistance, dyslipidaemia, asthma, bone metabolism or vitiligo. Regulation of the CAT gene expression plays an important role in the levels of CAT. The catalase gene expression is regulated by various mechanisms involving e.g. peroxisome proliferator-activated receptor γ (PPARγ), tumour necrosis factor α (TNF-α), p53 protein and hypermethylation of CpG islands in the catalase promoter. Transcription of the CAT gene is mainly influenced by the -262 C/T and -844 A/G polymorphisms. A common polymorphism -262 C/T in the promoter region has been found to be associated with altered CAT activities. Apart from genetic factors, the activities of CAT may be affected by age, seasonal variations, physical activity, or a number of chemical compounds. Future investigations are necessary to elucidate the role of CAT in pathogenesis of oxidative stress-related diseases.