[en] During 2006 and 2007 the Australian Nuclear Science and Technology Organisation (ANSTO) commissioned OPAL, a 20 MW open pool Research Reactor. This commissioning involved three stages: Stage A: testing the reactors systems prior to fuel loading; Stage B: first loading of fuel, achieving first criticality, reactor characterisation and systems testing up to 400 kw; and Stage C: raising the power of the reactor in steps, up to its full operating power of 20 MW. Prior to and following fuel loading a series of radiation measurements were made throughout the plant. These included dose rates, radioactivity in air, on surfaces and in cooling and shielding liquids. Installed continuous monitoring and portable equipment were used. Health physics measurements were repeated at increasing reactor power levels to check engineering design features and design of plant for radiation protection aspects. A Radiation Protection Plan and associated monitoring programs were implemented, including establishing and maintaining area, task and personnel monitoring regimes in the facility. Health Physics assessments and advice at each stage, played a major role in this commissioning process. This paper discusses the health physics experience of commissioning the OPAL Research Reactor and describes health physics results, actions taken and lessons learned during commissioning. (author)