Ocean harvests have plateaued worldwide and many important commercial stocks have been depleted. This has caused great concern among scientists, fishery managers, the fishing community, and the public. This book evaluates major models used for estimating the size and structure of marine fish populations (stock assessments) and changes in populations over time. It demonstrates that problems that may occur in fisheries data – for example, underreporting or changes in the likelihood that fish can be caught with a given type of gear – can degrade the quality of stock assessments. Recommendations are made for ways to improve stock assessments and their use in fishery management.