Noting that some public, school, law, academic, and special libraries have adopted marketing as a management tool, this paper begins by discussing marketing principles as defined by the American Marketing Association (AMA). Librarians are advised to center their marketing activities around what is known as the "4 P's" of marketing: Product, Price, Promotion, and Place. Other "P words" which libraries need to consider include Public Policy, Politics, and most importantly, People. It is argued that knowing one's patron/customer is not only the basis for good marketing, but it will determine success or failure for all institutions in the 1990s. It is predicted that the 1990s will test the skills of marketers as resources become scarcer and consumers more diverse in their interests and more particular about what they will purchase. It is concluded that librarians will need to have a clear understanding of their community's diverse needs in order to market their services successfully in the future. (MAB)