Hydrogen promises to provide some highly desired features for clean and efficient combustion, but harvesting efficiency and emission potentials as well as meeting engine durability requirements needs careful adaption of both, combustion system components and engine operation strategies. Key points for H2-ICE combustion are some specific and unique features of H2/air mixtures, among which – to name only a few – excellent dilutability, lean burn capability, low ignition energy and high molecular diffusivity and their consequences on ICE operation do play prominent roles. H2 admission via port or direct injection, compression ratio selection and injection timing provide a set of parameters to control combustion features. Cooling layout of cylinder head, liner and piston as well as heat rejection from components such as spark plugs, H2-DI injectors or valves must respect enhanced gas to component heat transfer in order to mitigate irregular combustion tendencies such as endgas knock and pre-ignition, and to ensure sufficient component durability. We apply well established pressure and flame analysis techniques to gain insight into the degrees of freedom for aspiration, H2 injection timing, compression ratio selection and heat transfer requirements. Cylinder pressure and visiolution flame sensors are applied in normal, multicylinder engine operation. This allows for stationary tests as well as for the cycle and crank angle resolved evaluation of entire (e.g. WLTP) test cycles. Application of such combustion diagnostic tools helps to exploit H2 admission and combustion options.