ABSTRACT. As the human immunodeficiency virus (HIV) pandemic progresses, an increasing number of recombinant viruses have been identified and in many geographical regions they are now the predominating strain. These recombinants are formed when an individual has acquired a co-infection or superinfection with more than one HIV-1 strain or subtype. Thus, dually infected individuals provide opportunities for studying HIV recombinants and viral interactions between infecting strains in vivo. The possible epidemiological, clinical and therapeutic implications of dual infections and recombination are many. Recombination may result in the emergence of more pathogenic and virulent HIV strains with altered fitness, tropism, and resistance to multiple drugs, and may hamper the development of subtype-based vaccines. This review is aimed at providing a more thorough understanding of dual infections (both co-infection and super-infection) and the possible consequences of the emergence of recombinant HIV-1 strains.