We describe a new software package capable of predicting the spectra of Solar system planets, exoplanets, brown dwarfs and cool stars. The Versatile Software for Transfer of Atmospheric Radiation (vstar) code combines a line-by-line approach to molecular and atomic absorption with a full multiple scattering treatment of radiative transfer. vstar is a modular system incorporating an ionization and chemical equilibrium model, a comprehensive treatment of spectral line absorption using a data base of more than 2.9 billion spectral lines, a scattering package and a radiative transfer module. We test the methods by comparison with other models and benchmark calculations. We present examples of the use of vstar to model the spectra of terrestrial and giant planet in our own Solar system, brown dwarfs and cool stars.