Libraries are service organizations that are facing increased competition from both public sector and commercial sector information service providers. This article first defines service quality. Then, it compares the approaches to measuring service quality used in libraries and in other service organizations. It suggests that one approach widely used in commercial service organizations -- looking at the differences between expectations and actual performance -- can be used effectively in libraries to provide diagnostic data for evaluating and modifying services. The article describes and assesses the findings of three studies that have applied SERVQUAL, an instrument widely used in business organizations, to judge the quality of library services (Hebert, 1993; White, Abels, and Nitecki, 1994; and Nitecki, 1995). These studies were done in public, special, and academic libraries respectively. The article concludes with an assessment of the appropriateness of the approach to measuring service quality and the appropriateness of SERVQUAL or the variant, Modified SERVQUAL, SLA Version, developed by White, Abels, and Nitecki to libraries. Appendix includes the Modified Servqual, SLA Version.