The invention relates to the 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 said process. The process offers the possibility to modify the NDV genome by means of genetic modification and allows the introduction of mutations, deletions and/or insertions. The process can be used to modify the virulence of NDV, thereby generating new attenuated live vaccines with enhanced properties. The process can be used to modify the antigenic make-up of NDV, thus allowing the generation of live NDV marker vaccines which can be serologically distinguished from NDV field strains.