For the last two decades, the development of the public library has been characterized by slogans like “from content to context”, “from collection to connection” or “from collection to creation”. All of which describe a development where the library has moved from a more classical focus on the collections to an increased focus on the users of the library and the surrounding community. While seize of the collections as well as lending figures are easily measureable the same is not the case when speaking of newer trends concerning the public library as a meeting space, a performative space, a learning space or as a broader cultural institution engaged in creating development in the community. On the background of the overall raison d’etre of the public library, resent trends and current library research, my contribution will put up a discussion of why the value of today’s public library is difficult to measure on the one hand and why measurements of value are so much in demand on the other. This will be done based on the so-called “Four-space model” and the work of library scholar R. David Lankes.