The poxviridae have a long history of causing disease in society, and their biological effects in humans and other mammals have been extensively studied. In the 1980s, genetic engineering techniques were applied to vaccinia in order to create replicating recombinant vectors that could express inserted genes encoding influenza virus proteins. In animal models, these recombinant viruses were able to deliver their foreign antigens to the immune system and elicit a specific adaptive immune response. Since then, improvements in our understanding of immunobiology, as well as technical advances in bioengineering, have led to the creation and clinical testing of a large number of recombinant poxviruses as candidate vaccines. Poxviruses can infect a broad range of cells, replicate with high efficiency and elicit strong immune responses - factors that make them especially well-suited as vaccines for the prevention and treatment of human immunodeficiency virus (HIV) and cancer. Both of these diseases are characterised by chronic antigen expression in the setting of focal or global deficits in the immune system that hamper the generation of protective immunity. This review traces the history of poxviruses as pathogens and immunogens, examines some of the approaches that have been taken to design poxviral vaccines for HIV and cancer and summarises the results of existing clinical trials of these vectors. In addition, the review aims to identify some of the factors that may shape the development of future therapies based on recombinant poxviruses.