Modified vaccinia virus Ankara (MVA) has become a widely used vector for vaccine and laboratory purposes. Despite significant advances in recombinant MVA technology, the isolation of recombinant viruses remains a tedious and difficult process. This chapter describes the use of an efficient and easy-to-use selection system adapted for MVA. The system is based on the requirement of the viral gene F13L for efficient virus spread in cell culture, which results in a severe block in virus transmission when F13L gene is deleted (Blasco R, Moss B. J Virol 65:5910-5920, 1991; Blasco R, Moss B. J Virol 66:4170-4179, 1992). The insertion of foreign genes in the MVA genome is accomplished by recombination of a transfected plasmid carrying the foreign genes and the F13L with the genome of an F13L knockout virus. Subsequently, selection of virus recombinants is carried out by serial passage and/or plaque purification of viruses that have recovered the F13L gene.