  We present an outline of basic assumptions and governing structural equations
describing atmospheres of substellar mass objects, in particular the extrasolar
giant planets and brown dwarfs. Although most of the presentation of the
physical and numerical background is generic, details of the implementation
pertain mostly to the code CoolTlusty. We also present a review of numerical
approaches and computer codes devised to solve the structural equations, and
make a critical evaluation of their efficiency and accuracy.
