The High Level Architecture (HLA) provides the specification of a common technical architecture for use across all classes of simulations in the US Department of Defense. It provides the structural basis for simulation interoperability. The baseline definition of the HLA includes the HLA Rules, The HLA Interface Specification, and the HLA Object Model Template. The HLA Rules are a set of 10 basic rules that define the responsibilities and relationships among the components of an HLA federation. The HLA Interface Specification provides a specification of the functional interfaces between HLA federates and the HLA Runtime Infrastructure. The HLA OMT provides a common presentation format for HLA Simulation and Federation Object Models.This paper provides a description of the development of the HLA, a technical description of the key elements of architecture, and a discussion of HLA implementation, including HLA supporting processes and software.