The Internet today is beset with constant attacks targeting users and infrastructure. One popular method of detecting these attacks and the infected hosts behind them is to monitor unused network addresses. Because many Internet threats propagate randomly, infection attempts can be captured by monitoring the unused spaces between live addresses. Sensors that monitor these unused address spaces are called darknets, network telescopes, or blackholes. They capture important information about a diverse range of threats such as Internet worms, denial of services attacks, and botnets. In this paper, we describe and analyze the important measurement issues associated with deploying darknets, evaluating the placement and service configuration of darknets, and analyzing the data collected by darknets. To support the discussion, we leverage 4 years of experience operating the Internet motion sensor (IMS), a network of distributed darknet sensors monitoring 60 distinct address blocks in 19 organizations over 3 continents.