Chronic granulomatous disease (CGD) is a rare inherited immunodeficency syndrome caused by a profound defect in the oxygen metabolic burst machinery. Activity of NADPH oxidase is absent or profoundly diminished, as at least one of its components (gp91phox, p22phox, p47phox and p67phox) is lacking or non-functional. This review explains the molecular basis of NADPH oxidase dysfunction by the effects of mutations in genes coding for particular oxidase components. Among the four types of CGD, the most common is X- -linked CGD (approximately 65%), with defects in the CYBB gene encoding gp91phox. A wide spectrum of mutations has been described in the CYBB gene with no predominant genotype. The second most common subtype of CGD caused by NCF1 mutation accounts for 30% of CGD patients and is inherited in an autosomal recessive manner, with predominance of a homozygotous .GT deletion in the genotype. The other two CGD subtypes having an autosomal recessive pattern together account for no more than 10% of CGD cases. A strategy for the molecular diagnostics in CGD patients is proposed and principles of genetic counseling are discussed here.