Abstract: A modular insulated building panel is disclosed. The panel is configured for connection to a like adjacently positioned insulated panel to form a building system. The panel comprises a central core of insulating material having two major surfaces and two side surfaces, substantially flat sheets on the major surfaces of the central core. Extending beyond the side surfaces of the core are male connectors having a generally convex curved first male mating member, a generally convex curved second male mating member, and a generally concave curved member between said first male mating member and said second male mating member. The panel further comprises female connectors having a generally concave curved first female mating member configured to accept the first male mating member, a generally concave curved second female mating member configured to accept the second male mating member, and a generally protruding member between the first female mating member and the second female mating member.