kCARTA is a pseudo line-by-line code that is the Reference Forward Model for the AIRS sounder on EOS-AQUA. The optical depths are rapidly computed from compressed lookup tables of pre-computed atmospheric molecular absorption coefficients, and can be used for any realistic nadir viewing infrared spectrometer. kCARTA can quickly compute Jacobians with respect to gas amount and temperature for (non-scattering) atmospheres. A fast hybrid 2-stream/adding-doubling algorithm has recently been added to kCARTA. In addition, two well known scattering packages, DISORT and RTSPEC, have been interfaced with kCARTA.