Star formation is one of the key pro­cesses governing the evolution of the Universe. It is ac­companied by significant transformations in the chem­ical composition and the physical state of the inter­stellar medium. Combining observations in infrared and radio bands, we obtain a wealth of observational data on the star formation both in continuum and spectral lines. These lines mostly belong to molecules, from simple diatomic species to complex organic com­pounds. However, interstellar molecules are not well- mixed with the bulk interstellar gas (molecular and atomic hydrogen) and possess a complex evolution of their own. Thus, a straightforward interpretation of observational data is seldom possible. One needs so­phisticated chemical and physical models to decipher processes occurring in star-forming regions.