Cometary Globules are small dense clouds commonly found in the surroundings of OB stars in HII regions. We present a detailed study, both numerical and analytical, of the "Radiation-Driven Implosion" model. It is shown that this model alone, based on the photo-ionisation by neighbouring stars, can account for the formation and the evolution of Cometary Globules. It is a two-stage process : a brief collapse phase, followed by the commonly observed and much longer cometary phase. Only the first stage has an unambiguous spectroscopie signature. We show that under some ionisation criteria small-scale and large-scale instabilities may grow at the cloud's surface, eventually leading to the cloud disruption. An analytic study shows that all clouds in the cometary stage are supported by a static magnetic field and are gravitationally stable. From simulations, we generated emissivity maps and position-velocity diagrams in order to allow a direct observational confrontation with our model. We observed CG7S, a cloud which appears to be in the pre-cometary stage. The kinematic properties of CG7S are simply explained by the Radiation-Driven Implosion model and well reproduced numerically
