A modus operandi (often shortened to M.O. or MO) is an individual’s habits of working, particularly in the context of business or criminal investigations, but also generally. It is a Latin phrase, approximately translated as 'mode (or manner) of operating'.^[1]

The term is often used in police work when discussing crime and addressing the methods employed by criminals. It is also used in criminal profiling,^[2] where it can help in finding clues to the offender's psychology.^[3] It largely consists of examining the actions used by the individuals to execute the crime, prevent its detection and facilitate escape.^[1] A suspect's modus operandi can assist in their identification, apprehension, or repression, and can also be used to determine links between crimes.^[4]

In business, modus operandi is used to describe a firm's preferred means of executing business and interacting with other firms.

The plural is modi operandi.^[5][6] The word operandi is a gerund in the genitive case, "of operating"; gerunds can never be pluralised in Latin, as opposed to gerundives. When a noun with an attribute in the genitive is pluralised, only the head noun normally changes, just as in English with "of": "a fact of life, two facts of life" (unlike, for instance, les modes opératoires in French).

• Criminology – Study of crime and criminal actions/behavior
• John E. Douglas – American criminal profiler
• Latin phrases
• Modus ponens – Rule of logical inference
• Modus tollens – Rule of logical inference
• Modus vivendi – Arrangement that allows conflicting parties to coexist in peace
• Signature crime – crime which exhibits characteristics unique to an offender's psychologyPages displaying wikidata descriptions as a fallback