This document summarizes a presentation on demand-driven acquisition (DDA) given at the Charleston Conference in 2011. It defines DDA and patron-driven acquisition (PDA) and discusses why libraries are adopting these models. Data is presented showing the decline in print book purchases and rise in ebook purchases. The presentation addresses challenges in DDA like availability of ebooks from publishers and how a multi-format DDA model could work to expand collections on demand within budget constraints.