This chapter contains sections titled: Introduction Amino Acid Sequence and Domain Structure of Retroviral Integrases Crystallization of Integrase Catalytic Domain of Integrase N-Terminal Domain of Integrase C-Terminal Domain of Integrase Two-Domain Constructs Consisting of N and Cat Domains Two-Domain Constructs Consisting of Cat and C Domains Oligomeric States of Full-Length Integrase and Modeling of Its Structure Structural Studies of Inhibitor Complexes of Integrase Structural Basis of Enzymatic Activity of Integrase Concluding Remarks Acknowledgment References