This invention concerns a security enclosure and in particular an outdoor, non-metallic security enclosure for electrical equipment. The enclosure has a body and a cover which is connected to the body using connecting means such that the cover is movable between a first, closed position wherein the security enclosure is closed and a second, open position wherein the security enclosure is open. The cover is lockable in its closed position. Complementary shaped engagement formations carried by the body and cover engage one another so as to create a mechanical seal between the body and cover when the cover is in its closed position. Retaining formations carried by the body and cover respectively are configured to prevent the cover from being removed from the body when the cover is in its closed position. The invention also concerns a security system including a number of security enclosures.