The molecular engineering of recombinant plasmid DNA clones containing the full-length and replication-competent feline foamy (retro)virus (FFV) proviral genome is described. The methods used to combine subgenomic FFV DNA fragments can be applied to other retrovirus genomes, resulting in full-length, bacterially cloned retroviruses. In addition, techniques used to determine the replication competence of the cloned viral genomes are described. Alternative technologies (not described here) are also discussed, together with the application of cloned genomes in modern molecular virology; for example, in functional genetic studies or as starting material for retrovirus vector development.