This thesis is concerned with the potential applications for in-cylinder ionisation probes for the feedback control of spark ignition engines. Such sensors are shown to yield useful qualitative information about the combustion process. Two different implementations of an in-cylinder ionisation probe are investigated - both types are demonstrated to have potential for specific control applications. The first implementation is a Flame front sensor - here an ionisation probe is used to determine the time of flame arrival at a position remote from the spark plug. This parameter is typically subject to a high degree of cyclic variability, but is generally sensitive to variables which affect the flame speed such as air/fuel ratio, turbulence characteristics etc. The flame arrival time is shown to be useful as an indicator of relative cylinder to cylinder variations. The general signal characteristics were determined for a range of engine conditions and a system for the real-time, feedback control of a fuel injection system was developed and demonstrated. Results showed that, with the controller implemented on a four cylinder engine, the lean misfire limit could be extended to higher air/fuel ratios and the brake specific fuel consumption was improved. The second implementation of a Post-flame ionisation sensor -- the residual ionisation in the burnt gases behind a flame front is used to provide a signal which is sensitive to cylinder temperature and pressure. The central electrode of the spark plug is conveniently located to produce such a signal -- providing precautions are taken to protect the signal circuitry from the high voltage ignition spark. The signal characteristics of the spark plug ionisation probe were evaluated. Applications of the signal to the feedback control of ignition timing and/or fuelling, based on the estimation of peak cylinder pressure arrival and knock intensity, are demonstrated.