Birth of protostars significantly affects chemical compositions of parent molecular clouds. Observational results of low-mass and high-mass star-forming regions are summarized, which show significant chemical differentiation within a source as well as among sources. In low-mass star-forming regions, hot corinos characterized by rich saturated organic molecules and warm carbon-chain chemistry sources characterized by rich carbon-chain molecules are known. In high-mass star-forming regions, complex chemical differentiation is seen in hot cores around protostars. Thermal evaporation of grain mantle plays a crucial role in chemistry of star-forming regions. Shocks associated with outflows and accretion motions also contribute to chemical differentiation. Furthermore, photodissociation regions illuminated by young stars and nearby stars reveal unique chemistry. Key physical and chemical processes in the star-forming regions are described.