Janus presided over the beginning and ending of conflict, and hence war and peace. The doors of his temple were open in time of war, and closed to mark the peace. As a god of transitions, he had functions pertaining to birth and to journeys and exchange, and in his association with Portunus, a similar harbor and gateway god, he was concerned with travelling, trading and shipping.

Janus had no flamen or specialised priest (sacerdos) assigned to him, but the King of the Sacred Rites (rex sacrorum) himself carried out his ceremonies. Janus had a ubiquitous presence in religious ceremonies throughout the year, and was ritually invoked at the beginning of each one, regardless of the main deity honored on any particular occasion.

The ancient Greeks had no equivalent to Janus, whom the Romans claimed as distinctively their own.