The present invention relates to a cytomegalovirus (CMV) which has been recombinantly altered to express a heterologous polypeptide and to allow for external control of viral replication. The heterologous polypeptide may be a polypeptide of interest such as an antigen, antibody or immune modulator. The CMV vectors of the invention are replication defective, or chemically controllable replication capable, or replication competent. The present invention also relates to uses of the CMV vectors such as inducing an immune response to an antigen or expressing an antibody or immune modulator in vivo. Compositions comprising the CMV expressing the heterologous polypeptide are also encompassed by the present invention.