Stars like our sun and planets like our Earth form in the dense interstellar medium (ISM). The dense ISM also shields molecules from the interstellar radiation field and allows a rich chemistry to occur. More than 150 different gas phase molecules and around 20 molecular species on the grain surface have been detected in the various regions of the ISM. Many of these molecules are organic, and therefore important astro-biologically. These molecules range in complexity from diatomic H2 to a 15-atom linear nitrile, HC13N and many of these molecules are quite unusual to find in ISM by terrestrial standards. There is a wide variety of processes that can lead to the formation of these molecules in star forming regions. These can be divided into two major classes: the reactions that can occur through gas-phase chemistry, and the reactions that occur on the surfaces of interstellar grains. Numerical techniques developed to study the formation of these molecules include the rate equation method, as well as several more detailed stochastic methods, based upon either the direction solution of the master equation or a Monte Carlo realization of the problem. In this talk, I will present results obtained for diffuse clouds, grain mantle compositions and their dependence on various physical parameters associated with a star forming region. I will also discuss the effect of grain growth and grain size distribution on chemical abundances and the modeling of O2 and H2O in IRAS 4A.