Bacteria in the genus Streptomyces are celebrated for their metabolic sophistication. Many clinically used antibiotics, anticancer and other bioactive agents are derived from Streptomyces spp or closely related Actinobacteria. So why bacteriophages? As studies on phage  show, the virus – host interaction is intimate and investigations on phage development can lead to new insights into the physiology of the host. On this theme our quest to discover the nature of the phage C31 receptor in Streptomyces coelicolor led to a description of a protein glycosylation pathway required for membrane integrity and innate resistance to some antibiotics. Furthermore host - phage interactions and the components required for phage development have led to a plethora of tools for the genetic manipulation of their hosts. Researchers are Eli Lilly pharmaceutical company were developing a new type of vector for use in Streptomyces based on the integration functions of C31 integrase. This unusual integrase intrigued us, so we set to work trying to understand its mechanism and to provide further tools for genetic manipulation of Streptomyces spp and their close relatives. We discovered that the C31 integration system, required for establishment of the integrated prophage, was composed of just three elements: two approximately 45 bp recombination substrates and the integrase itself. Depending on how the recombination sites were arranged in the DNA, the result was predictable and irreversible DNA rearrangements. These properties meant that the C31 recombination system offered some advantages over other site-specific recombination tools for use in heterologous organisms. C31 excision, during which the phage genome is excised from the host chromosome, requires an additional phage protein. We now understand in some detail how C31 and related phage integrases work and control their directionality.