The Return-On-Investment (ROI) rate, when determined and positively proven, may be the most effective tool for marketing a library. ROI is a cost-and-benefit analysis that determines whether money spent on the library produces a positive return. Proving and communicating a library's value to its governing board or constituents is increasingly necessary for law libraries to survive and flourish. A ROI analysis may be difficult to undertake if the library works in a not-for-profit setting, and requires a set of criteria different from those used for libraries in the for-profit setting. Although the ROI is a time-consuming and difficult analysis, libraries should take advantage of this instrument.