When may a police officer arrest someone for a misdemeanor?

To make a summary arrest for a misdemeanor offense, GENERALLY, the officer must actually observe it take place (i.e.: it occurs within range of his senses).

HOWEVER: Many(all?) jurisdictions list exceptions to this which involve more serious offenses, and which are known as "Probable Cause Misdemeanors," and an officer may make an arrest even though he may not have actually observed the crime.

One jurisdiction's list (Washington DC) is typical: "Simple Assault - Petit Larceny - Unlawful Entry - Receiving Stolen Property - Attempted Burglary - Attempted Larceny - Attempted Auto Theft." The foregoing seven offenses are all codified as Misdemeanors, however they are the listed exception to the misdemeanor arrest rule, and an officer need not have observed them take place in order to make a summary arrest.

CAUTION: Other jurisdictions exceptions MAY be different.