  We present Korg, a new package for 1D LTE (local thermodynamic equilibrium)
spectral synthesis of FGK stars, which computes theoretical spectra from the
near-ultraviolet to the near-infrared, and implements both plane-parallel and
spherical radiative transfer. We outline the inputs and internals of Korg, and
compare synthetic spectra from Korg, MOOG, Turbospectrum, and SME. The
disagreements between Korg and the other codes are no larger than those between
the other codes, although disagreement between codes is substantial. We examine
the case of a C$_2$ band in detail, finding that uncertainties on physical
inputs to spectral synthesis account for a significant fraction of the
disagreement. Korg is 1-100 times faster than other codes in typical use,
compatible with automatic differentiation libraries, and easily extensible,
making it ideal for statistical inference and parameter estimation applied to
large data sets. Documentation and installation instructions are available at
https://ajwheeler.github.io/Korg.jl/stable/.
