N or M? is a detective novel written by Agatha Christie in 1941. It is one of five books written by Christie in which the Beresfords (Tommy and Tuppence) are the detectives.

Set in the beginning of World War II, the story opens with Tommy Beresford being approached by a British agent, Mr. Grant. Grant informs Tommy that certain Germans have infiltrated the British chain of command. Another British agent that was following these Germans left a criptic message on his deathbed: "N or M. Song Susie". Grant knew that "Song Susie" stood for Sans Souci, a hotel in Leahampton, and N and M were two German spies. Tommy is to go to Sans Souci to investigate whether N, M or both are at the hotel and to figure out their identities. Not to be outdone, Tuppence finds a way to join Tommy at Sans Souci and together they attempt to solve the mystery.