Pathogenicity islands (PAIs) are a group of mobile genetic elements that play a pivotal role in the virulence of bacterial pathogens of humans. Typical PAIs are distinct regions of DNA that are present in the genome of pathogenic bacteria. PAIs are mostly inserted in the backbone genome of the host strain at specific sites. Pathogenicity islands are considered as a subset of horizontally-acquired genomic islands. Acquisition of PAIs by horizontal gene transfer (HGT) is an important mechanism in the development of disease-causing capability and the evolution of bacterial pathogenesis. Protein secretion systems encoded by PAI include 5 different types of secretion systems: type I, II, III, IV and V which are general requirements for pathogenic and nonpathogenic bacteria. Pathogenicity islands have been described in different Gam-negative and Gram-positive bacteria. The acquisition of knowledge about PAIs, their structure, mobility and pathogenicity factors not only is helpful in gaining a better understanding of bacterial evolution and pathogen interaction with eukaryotic host cells.