Interstellar HII regions are formed through joint effects of the wind and ionizing radiation produced by one or more early type stars. In the early phase of HII region, a rapid R-type ionization front propagates out to the Stromgren radius. This ionisation front typically drives a shock into the initial cloud. The clouds will be compressed by the ionisation-shock front and eventually star formation takes place. This process is termed as 'Radiation Driven Implosion' and is considered to be responsible for sequential star formation. The way how the ionisation-shock front propagates into an initially spherical cloud and how star formation takes place in the RDI mode will be explained in the talk.