Empirical evidence has demonstrated that fish experience pain, and so to ensure their good welfare, it is vital that we can recognize and assess pain. A range of general, behavioral, and physiologic indicators can be used when assessing pain in fish. Many of these can be used at the tank side and are termed operational welfare indicators, whereas some require further computer or laboratory analysis. Behavioral indicators are valid and have been shown to profoundly differ between nonpainful and painful treatments in fish. However, these are not universal, and species-specific differences exist in behavioral responses to pain.