The stringent emission regulations and global need for decarbonization are driving the current engine research toward cleaner fuels. Hydrogen internal combustion engines are considered a promising solution for decarbonizing difficult-to-electrify sectors such as heavy-duty transport, agriculture, marine propulsion, construction machinery, and mining. Converting existing diesel engines to spark-ignition engines by modifying the fuel delivery and ignition systems enables the use of hydrogen as a fuel, resulting in near-zero CO2 emissions. Further, hydrogen’s wide flammability range allows the leaner operation achieving both higher engine efficiency and near-zero NOx emissions. This chapter discusses lean hydrogen combustion in a modified spark-ignition optical engine, using both port-fuel and direct injections. In-cylinder pressure analysis and high-speed visualization provide insights into combustion modes, aiding numerical model development and validation.