The aim of this paper is to demonstrate the performance of micro-invasive optical diagnostics as advanced tools in the development process of modern direct-injection (DI) gasoline engines. The use of endoscopes and optical probes minimise the mechanical modifications on the engine necessary to achieve the optical access to the combustion chamber. No expensive optical engines with large optical windows are required but only small holes of about 10 mm in the cylinder head or in a plate between cylinder head and cylinder liner are used to apply laser diagnostics. Basic in-cylinder phenomena, such as the formation of the flow field, the penetration of the spray at high fuel pressure, the interaction of spray and flow, the formation of an ignitable mixture and the start of combustion are analysed in detail. High-power solid-state pulsed lasers emitting ultraviolet or green light, state-of the-art high-speed colour video cameras, and newly designed optical probes were used for the investigations. Selected results from current research and development work demonstrate the capability of micro-invasive techniques and pinpoint how the design of the combustion process benefits from these experimental investigations.