The High Level Architecture (HLA), which has been accepted as an IEEE standard, is the specification for distributed interactive simulation. Along with the scope and scale of the HLA distributed simulation applications expanding, Verification, Validation and Accreditation (VV&A) has been an important research aspect of HLA distributed simulation domain. The main research goal of VV&A is to increase the credibility of modeling and simulation (M&S). The dissertation focuses on the key technologies of VV&A research in HLA simulation systems, with emphasis on the theories of process model, conceptual model validation, data Verification, Validation and Certification (VV&C) and accreditation. It also analyses the method, tool and application of VV&A in HLA simulation systems.Firstly, the dissertation introduces the HLA Federation Development and Execution Process (FEDEP). Based on the basis of HLA FEDEP, it puts forward one VV&A process model of FEDEP and analyses VV&A activities, products and management in each step. It also gives the description method of the process, and describes the process with IDEFO method. To increase efficiency and decrease risks, the dissertation puts forward several approaches to select VV&A activities.Secondly, there exist some theory problems affecting federation credibility in the whole VV&A process. The dissertation discusses several key points of the conceptual model validation problem. It focuses on warfare models and puts forward a process maturity model of warfare model validation. Data is important in M&S. The dissertation does research on data verification, validation and certification. Based on the research, it describes how to increase data credibility. Then the dissertation discusses fidelity of HLA simulation systems. It gives a fidelity framework supporting interoperability in HLA distributed simulation systems. It also studies the accreditation problem of federations.Thirdly, method is the core of HLA VV&A. The dissertation introduces methods of each stage in general M&S VV&A process. It discusses method architecture of HLA VV&A. In particular it analyses the characteristics of HLA simulation systems. The dissertation puts forward object-oriented testing method and fuzzy logic method. These methods can apply better in VV&A of HLA simulations. It also discusses the verification of DEVS/HLA federation.Finally, the dissertation studies tools, testing and application of HLA simulation systems in details. Based on the analysis of VV&A process, the dissertation discusses VV&A tools in HLA simulation systems. This will promote the automation degree of the whole VV&A activities. The dissertation develops federation verification tool (KD-FVT). The software has been tested and will be applied in real federations. Run-Time Infrastructure (RTI) is the coreof HLA. The dissertation performs functional tests and capability tests on RTI. It can make the RTI perfect. The dissertation also applies above research results to an actual HLA simulation system.