The invention provides a method of treating cancer in a mammal comprising administering to the mammal an effective amount of virus, particularly Newcastle Disease Virus or other Paramyxovirus. The invention also provides a method of treating cancer in a mammal comprising administering such viruses to the mammal in combination with another agent such as a chemotherapeutic compound, immunoadjuvant, cytokine, or immunosuppressive agent. The invention further provides a method of detecting cancer cells in a mammal using Paramyxovirus as an imaging agent and as an indicator of cancer cell growth in the mammal. The invention further provides genetically engineered Paramyxoviruses, and kits containing the viral compositions disclosed by the invention.