Return on investment (ROI) is a ratio used to evaluate the efficiency of an investment or to compare the efficiency of a number of different investments. The document discusses how ROI can be used in library collection development to justify budget allocations, evaluate the success of previous acquisitions, and demonstrate the value and impact of the collection. Some challenges to using ROI include a lack of consensus on measurement values and difficulties assessing intangible values. The document also reviews previous research on applying ROI in libraries and factors that influence its use.