Homogeneous charge compression ignition (HCCI) combustion, also known as controlled autoignition (CAI) combustion, has been realized over a range of engine speed and load conditions in a single cylinder four-stroke gasoline engine equipped with a 4VVAS mechanical variable intake and exhaust valve lift and timing devices system. The engine has the capability to operate either in the spark ignition (SI) or HCCI combustion in order to cover the complete engine operational range. Therefore, smooth switching between SI and HCCI modes is required. In this work, a systematic investigation has been carried out to study the transition between the conventional spark ignition combustion and HCCI combustion and develop the control strategies for optimized mode switch. Results show that dynamic transitions between HCCI and SI can be stably achieved around the boundaries of HCCI operation region through the rapid and effective control of the in-cylinder residual gas concentration by a Four Variable Valve Actuation System (4VVAS). It is found that switching from HCCI to SI operation is less problematic than that from SI to HCCI combustion operation, and certain regions of residual concentrations should be avoided. During the switching process, the spark timing and exhaust valve closing timing have the largest effect and can be optimized for mode switching. Two major approaches have been developed and applied to controlling the HCCI-SI transitions, through the dynamic management of residual gas and the management of hybrid heat release processes.