Learning is often identified with the acquisition and encoding of new information. Reading a textbook, listening to a lecture, participating in a hands-on classroom activity, and studying a list of words in a laboratory experiment are all clear examples of learning events. Tests, on the other hand, are used to assess what was learned in a prior experience but are not typically viewed as learning events. The act of measuring knowledge - by recalling or recognizing items, by answering questions, or even by retrieving and applying knowledge to solve novel problems - is not thought to change knowledge, just as measuring one's height would not make one taller and measuring one's weight does not leave a person lighter. [This chapter was published in J. H. Byrne (Ed.), "Learning and Memory: A Comprehensive Reference, 2nd edition" (p487-514), Cambridge, MA: Elsevier. The James S. McDonnell Foundation provided support for this research.]