This chapter describes the development of recombinant oncolytic measles viruses (MeV) that selectively enter and destroy tumor cells. The envelope of MeV is a favorable targeting substrate because receptor attachment and membrane fusion functions are separated on two proteins: the hemagglutinin (H) that binds receptors, and the fusion (F) protein that fuses the viral envelope with the cell membrane. The cell entry process, which depends on receptor recognition and occurs at the plasma membrane at neutral pH, results in the delivery of encapsidated genomes to the cytoplasm, where they replicate. Towards improving cancer specificity of oncolytic MeV, two types of cell entry targeting have been achieved. First, entry has been redirected through cancer-specific cell surface proteins. This was done by displaying specificity domains on H while also ablating binding to its natural receptors. Second, activation of the F protein was made dependent on secreted cancer proteases, while also interfering with F cleavage/activation by a ubiquitous intracellular protease. This chapter describes how entry-targeted MeV are produced: In short, gene cassettes with modified H or F coding regions are generated, and then introduced into the viral genome available on plasmid DNA. Such full-length genome plasmids are transfected in cell lines expressing, stably or transiently, the three viral proteins necessary for genome replication. Infectious centers form among these “rescue” cells, which allow isolation of clonal recombinant viruses. These are amplified, characterized in vitro, and then evaluated for their oncolytic activity in appropriate preclinical animal models.