  The large quantity and high quality of modern radio and infrared line
observations require efficient modeling techniques to infer physical and
chemical parameters such as temperature, density, and molecular abundances. We
present a computer program to calculate the intensities of atomic and molecular
lines produced in a uniform medium, based on statistical equilibrium
calculations involving collisional and radiative processes and including
radiation from background sources. Optical depth effects are treated with an
escape probability method. The program is available on the World Wide Web at
http://www.sron.rug.nl/~vdtak/radex/index.shtml . The program makes use of
molecular data files maintained in the Leiden Atomic and Molecular Database
(LAMDA), which will continue to be improved and expanded. The performance of
the program is compared with more approximate and with more sophisticated
methods. An Appendix provides diagnostic plots to estimate physical parameters
from line intensity ratios of commonly observed molecules. This program should
form an important tool in analyzing observations from current and future radio
and infrared telescopes.
