Studies of the star formation process have concentrated, on the one hand, on determining the density and kinematics of the cold interstellar medium and, on the other hand, on understanding and observing the physics of protostellar sources when the accretion energy of the protostellar core makes them detectable in the near infrared.
It has become understood in the last few years that turbulence and magnetic fields stabilize the cold interstellar medium on large scales and that gravitational instability begins on small scales. Nevertheless the complicated interplay between gravity, supersonic turbulence, magnetic field and thermodynamics of the gas is poorly understood and there is, as yet, no theory that can predict with any accuracy the star formation rate as a function of a few critical parameters of the interstellar clouds.
To understand the physical mechanisms which control star formation, detailed investigations are required of the critical phase when a small region of the ISM becomes gravitationally unstable. The detection with large ground-based millimeter and sub-millimeter telescopes of a small number of dense and cold condensations in star forming regions (Mezger 1994) provides a few tentative glimpses of the onset of gravitational instability. Nevertheless, large scale surveys are impossible with these techniques. PLANCK will provide the first systematic search for cold condensations in the ISM over a large fraction of the Galaxy and will provide a unique way of finding regions forming cold dense cores which have not yet produced stars detectable in the near infrared.