Retroviruses arguably belong to the most fascinating of all viruses because of their unusual and highly efficient mode of replication involving reverse transcription and integration of the viral genome and a complex system of transcriptional and post transcriptional regulatory mechanisms. The importance of ret roviruses as human and animal pathogens has also enhanced scientific and medical interest in this diverse group of viruses and has spurred an intensive search for novel and improved antiviral agents. More recently, analysis of retroviral replication and in particular understanding the formation and composition of the virus particle has received additional attention because of the promise of retroviral vectors as vehicles for human somatic gene therapy. Many recent advances have been made in our understanding of the molecular mechanisms governing as sembly and release of infectious retrovirus particles. This book attempts to summarize these recent developments and to provide an overview of our current knowledge on retrovirus particle formation. The individual chapters of the book deal with specific steps in the pathway of retroviral morphogenesis and maturation, starting at the time when the components of the virus have been synthesized within the infected cell and ending once the infectious virion has been released from the cell. An introductory chapter provides a comparative description of the structure and morphology of various retroviruses.