This manual identifies a series of performance measurements which, when assessed in their totality, can effectively measure a public library's level of service to its users. The measures have been developed at two levels. On one level, the measures can broadly evaluate the overall performance of library systems on the national level; on a second, more detailed level, the measures can be developed to assess an individual library's performance. Mathematical formulas provide a means of measuring the library's level of service in terms of its books, periodicals, sound and video recordings, pictures in stock, annual additions, and turnover rate; professional and para-professional staff availability; total floor area of building, library business hours, geographical accessibility; loans and circulation rate, library visits, percentage of lending service, attendance at library activities, reference and information transactions, user satisfaction with library stock, requests, and reservations. Ways in which consumer opinion can be surveyed are discussed, and guidelines are provided for adapting the measures to the particular situation and interpreting the results. An eight-item selected bibliography and a glossary of terms and definitions conclude the manual. (MAB)