The invention relates to a process for generating infectious Newcastle disease virus (NDV) entirely from cloned full-length cDNA and to the use of vaccines and diagnostic assays generated with and derived from the process. The process offers the possibility to modify the NDV genome by means of genetic modification and allows for the introduction of mutations, deletions and/or insertions. The process can be used to modify the virulence of NDV, thus generating new attenuated live vaccines with enhanced properties. The process can be used to modify the antigenic make-up of NDV, to allow the generation of live NDV marker vaccines that can be serologically distinguished from NDV field strains.