Although it is estimated that 20-30% of the general human population are carriers of Staphylococcus aureus, this bacterium is one of the most important etiological agents responsible for healthcare-associated infections. The appearance of methicillin resistant S. aureus (MRSA) strains has created serious therapeutical problems. Detailed understanding of the mechanisms of S. aureus infections seems necessary to develop new effective therapies against this pathogen. In this article, we present an overview of the biochemical and genetic mechanisms of pathogenicity of S. aureus strains. Virulence factors, organization of the genome and regulation of expression of genes involved in virulence, and mechanisms leading to methicilin resistance are presented and briefly discussed.