 Currently, there are a few dozen state-of-the-art GCMs/ESMs housed at modeling centers around the world (see Flato et al. 2013: Table 9.1). These are huge models, in some cases involving more than a million lines of computer code; it takes considerable time to produce simulations of interest with them, even on today’s supercomputers. To construct a GCM/ESM from scratch requires a vast range of knowledge and expertise (see Lahsen 2005). Consequently, many of today’s GCMs and ESMs have been built upon the foundations of earlier generations of models (Knutti et al. 2013). Such models often have a layered history, with some parts of their code originally developed years or even decades ago—sometimes by scientists who are no longer working at the modeling center—and other parts just added or upgraded. Even models at different modeling centers sometimes have pieces of computer code in common, whether shared directly or borrowed independently from an earlier model.