Upon entry of a retrovirus into the cytoplasm of a permissive cell, a viral protein, reverse transcriptase, generates cDNA using viral genomic RNA as template. In a subsequent but also essential step in the retroviral replication cycle, the nascent viral cDNA is ligated to chromosomal DNA by another viral protein, integrase, establishing the provirus. Retroviral infection is thus mutagenic to the host. One highly publicized illustration of the potential for retroviruses to disrupt host gene expression involved activation of the LMO2 proto-oncogene by proximal integration of a vector in a gene therapy trial for X-linked severe combined immunodeficiency (33). Aside from altering host gene expression, retroviruses such as human immunodeficiency virus type 1 (HIV-1) may cause pathology by transducing toxic genes (24). In addition to the genes encoding structural proteins and enzymes, which are themselves toxic to host cells, HIV-1 possesses six accessory genes that exhibit wide-ranging effects on cell physiology. Despite immune responses that decrease HIV-1 viremia after acute infection and appear to limit viral pathology for a decade, HIV-1 ultimately kills the host, most likely via complex effects involving all nine of the transduced viral genes. Given the potent effects of retrotransposition on biology and evolution, it is not hard to imagine that susceptible host organisms would elaborate factors that block reverse transcription or any of the steps that lead to integration. Several such factors have been identified and have been discussed previously in a comprehensive fashion (7, 31). This review will focus on how, in the course of efforts to identify retroviral inhibitory factors, cyclophilin A (CypA) was discovered to be an HIV-1 CA-binding protein. It will then describe how the characterization of the HIV-1 CA-CypA interaction revealed TRIM5 to be a potent antiretroviral restriction factor. Finally, it will present our current understanding of the mechanism of retroviral inhibition by TRIM5 and explain how CypA modulates retroviral restriction activity.