Sodankylä Ion Chemistry (SIC) model is a detailed ion-chemical scheme of the D region, in a conceptually simple model. Today, the model is accepted as one of the standard ion chemistry models at physics handbook level. The SIC model was originally built to be a tool for interpretation of D-region incoherent scatter experiments and cosmic radio noise absorption measurements, both of which form the basis of D-region research done at Sodankylä Geophysical Observatory. The number of different ions in the current version is 55, of which 36 are positive and 19 are negative. The model solves for ion and electron concentrations in ionospheric D and lower E regions either as a steady state model or a time dependent model assuming a predescribed neutral atmosphere. Applications of the model have been case studies to determine neutral NO concentration, neutral temperature, and negative ion properties in incoherent scatter and heating experiments. The model was also used to invert measured electron concentration profiles to parameters of the precipitating electron spectra. Additionally model profiles of electron concentration were used in theoretical estimates on the effect of possible greenhouse cooling of the upper atmosphere on the absorption of cosmic radio noise. Recent applications include modeling the response of atmospheric composition to local precipitation of auroral electrons as well as the response of the composition to solar proton events.