The rise of computer-based testing has brought with it the capability to measure more aspects of a test event than simply the answers selected or constructed by the test taker. One behavior that has drawn much research interest is the time test takers spend responding to individual multiple-choice items. In particular, very short response time--termed "rapid guessing"--has been shown to indicate disengaged test taking, regardless whether it occurs in high-stakes or low-stakes testing contexts. This article examines rapid-guessing behavior--its theoretical conceptualization and underlying assumptions, methods for identifying it, misconceptions regarding its dynamics, and the contextual requirements for its proper interpretation. It is argued that because it does not reflect what a test taker knows and can do, a rapid guess to an item represents a choice by the test taker to momentarily opt out of being measured. As a result, rapid guessing tends to negatively distort scores and thereby diminish validity. Therefore, because rapid guesses do not contribute to measurement, it makes little sense to include them in scoring.