There are many suggested motives for spying that an individual may have. In general, espionage carries heavy penalties, with spies often being regarded as traitors, and so motivating factors must usually be quite large.

There have been various attempts to explain why people become spies. One common theory is summed up by the acronym "MICE", which stands for "Money, Ideology, Compromise or Coercion (depending on source), and Ego" or Extortion (depending on source). Other explanations have stressed the role of disaffection and grudges, or of personal links.