This document discusses modeling energy technology cost reductions using learning curves. It begins by introducing single-factor learning curves that relate cost reductions to deployment levels. However, these cannot explain differences between emerging and current technologies or various drivers of global versus local learning. The document then reviews literature using multi-factor learning curves that incorporate additional drivers such as R&D, supply chain interactions, economies of scale, and component-level costs. These provide a more comprehensive understanding of innovation challenges but require more data and have limitations from model complexity and variable correlation. In conclusion, multi-factor learning curves are an improved approach for analyzing technology cost reductions.