The Paramyxoviridae family has several genera that include important human and veterinary pathogens such as Rubulavirus, Respiroviruses, Henipavirus, and the Avulavirus genus that contains Newcastle disease virus (NDV) and other avian paramyxoviruses (APMV). This chapter focuses on infections of poultry with NDV. It offers detailed coverage of the history, etiology, pathobiology, epidemiology, diagnosis, and intervention strategies of Newcastle disease, APMV, and avian Metapneumovirus Infections. AMPV infections continue to emerge as a disease threat with four defined subtypes, A-D, being recognized and producing clinical disease in both turkeys and chickens. For effective disease management, it is important to be able to identify birds that are infected with NDV and distinguish vaccine viruses from virulent viruses. Regardless of whether ND control is applied at the international, national, or farm level, the objective is either to prevent susceptible birds from becoming infected or to reduce the number of susceptible birds by vaccination.