Bayesian Atmospheric Radiative Transfer (BART, Harrington et al. 2021, Cubillos et al. 2021, Blecic et al. 2021) is an open-source, reproducible-research code for atmospheric composition and structure retrieval. Its modular design and high degree of user control allow it to be configured to mimic the physics hardwired into many other codes. Users can configure BART to reproduce published work and then explore the effect of different choices for line lists, included species, thermal-profile models, Bayesian samplers, and, shortly, cloud parameterizations, among others. By implementing the approaches of differing authors, BART has resolved controversies. BART comes with its own verification test package, BARTTest, which can be applied to any radiative-transfer or retrieval code. In developing BART, we learned valuable lessons in statistics and spectral modeling, which we will also present. This research was supported by the NASA Fellowship Activity under NASA Grant 80NSSC20K0682 and NASA Exoplanets Research Program grant NNX17AB62G.