Modus operandi (often used in the abbreviated form MO) is a Latin phrase, approximately translatable as "mode of operation." It is used in police work to describe a criminal's characteristic patterns and style of work. The term is also commonly used in the United States of America in a non-criminal sense to describe someone's habits.

A modus operandi can be used to narrow down an investigation amongst a group of criminals. For example, a burglar may typically use a back window, broken with a rock. If one of a group of known criminals is suspected to have committed a burglary, then their characteristic MOs can be compared to help identify the burglar.

A criminal's MO can also be used in profiling , where it can be used to find clues to the perpetrator's psychology.