Public libraries deliver many benefits to their patrons, but understanding these benefits in terms of dollars-and-cents is difficult. In an effort to quantify the return on investment to taxpayers for monies invested in public libraries, the Library Research Service (LRS) initiated the study, "Public Libraries -- A Wise Investment: A Return on Investment Study of Colorado Public Libraries," in May 2006. This research utilized a multiple case-study approach, examining data from eight Colorado public libraries that represented economically and geographically diverse regions of the state. Participating libraries included Cortez Public Library, Denver Public Library, Douglas County Libraries, Eagle Valley Library District, Fort Morgan Public Library, Mesa County Public Library District, Montrose Library District, and Rangeview Library District. Data were gathered using a combination of questionnaires, available data sources, and key informant interviews. Nearly 5,000 Colorado residents responded to the questionnaire; in addition, library staff and community members were interviewed as key informants about library services and their economic value. Available data on library staff expenditures and library spending with vendors and contractors were obtained from participating libraries. The results of the study indicate that for most libraries, the return on investment for Colorado public libraries is approximately 5 to 1--that is, for every $1 spent on public libraries, $5 of value is realized by taxpayers. An analysis of the many ROI studies conducted nationwide reveals that the differences in their resulting ROI ratios are readily understood by considering what returns and which investments are included as well as which services are addressed and which "market value" multipliers are used. An analysis of patterns in various types of library use reported by respondents for participating libraries also revealed notable variations. Generally, it appears that the library--whether it is located in a metropolitan or non-metropolitan area and whether it is a central city or a suburb--is strongly associated with these patterns (e.g., higher circulation rates for non-print formats, such as audio books and DVDs, greater use of library computers to access Web resources). Appended are: (1) Individual Library Reports; (2) Return on Investment Demographic Analysis--Selected Findings; (3) Library Survey; (4) Patron Survey; and (5) Key Informant Interviews. (Contains 174 charts and 27 tables.) [This report was produced by Library Research Service.]