Methods for optical access to combustion chambers of internal combustion engines are shown. The optical access is either maximized to allow application of complex optical diagnostic techniques while maintaining minimum necessary operability of the engine or engine components, or full engine operability is maintained while optical access and diagnostic techniques are tailored to the diagnostic demand and the restraints of engine operation. As full optical access allows a wide range of diagnostics to be applied, it puts the emphasis on organizing and extending realistic engine operation conditions. Full engine operability, however, challenges the optical access and the design of diagnostic systems. The paper gives examples for diagnostic applications for both of these situations.