1. Why Do We Measure Stress?2. Quantifying Stress3. Specific Measures of Fish Stress 3.1. Cellular and Molecular Indicators3.2. Primary and Secondary Physiological Indicators3.3. Whole-Organism Indicators4. Considerations for Measuring and Interpreting Stress 4.1. Interspecific Differences4.2. Intraspecific Differences4.3. Context-Specific Differences4.4. Stressor Severity4.5. Field Versus Laboratory4.6. Temporal Aspects5. From Individual Indicators to Ecosystem Health6. Stress Indicators of the Future7. ConclusionA fish is chased with a net in an aquarium before being captured, scooped out of the water, and placed in a nearby testing arena. Is it stressed? How can we tell? Are our indicators reliable? Quantification of stress in fish has evolved from the initial development of radioimmunoassays to measure cortisol in plasma to the rapidly expanding suite of genome-based assays. Indicators range from the intracellular to whole-animal level. Expression of heat shock proteins (HSPs) and activity of metabolic enzymes can be paired with straightforward observations of reflexes and survival. Both traditional and emerging indicators have advantages and disadvantages, and their use is tissue- and context-specific. Ecological, biological, and methodological factors must be considered when selecting, measuring, and interpreting stress indicators. Inter- and intraspecific, sex, life stage, and temporal differences in physiological responses to stressors can confound confirmation of a stressed state. Despite numerous types of indicators, our understanding of how absolute levels of indicators relate to stressor severity and recovery to date remains limited. How accurately indicators characterize stress in wild populations naturally exposed to stressors is still an evolving discussion. The integration of research disciplines and involvement of stakeholders and user groups will aid in filling these knowledge gaps, as well as the translation of individual-level indicators to population- and ecosystem-level processes.