This publication presents the findings of an IAEA coordinated research project (CRP) focusing on validation of the safety and operational aspects of high temperature gas cooled reactors (HTGRs) under projected and actual operating conditions. Specifically, it documents the results of a benchmark analysis of the ASTRA critical facility at the Kurchatov Institute in the Russian Federation, with respect to the development of pebble bed high temperature modular reactor (PBMR-400). It also presents results of benchmark analyses performed for the HTR-10 experimental reactor in China, and the pebble bed micromodel test facility in South Africa. Such benchmarks include core height prediction for criticality, control rod worth and related differential reactivity and interference coefficients, and investigation of critical parameters for differing heights of the pebble bed high temperature modular reactor. Code to code comparison as well as comparison with actual experimental data make the information obtained from this CRP valuable for verification and validation of HTGR design and analysis codes for future developers of HTGR power plants. General results and conclusions as delineated by the participating Member States are described in the summary.