This chapter introduces principles for welfare assessment of farmed fish. The introduction details the developments in science and society regarding the proposition that fish have a welfare state and that welfare assessment is important. The next sections of the chapter highlight the neurological basis of fish welfare and outline the primary welfare needs of the fish; appropriate water environment, adequate nutrition, good health, the expression of (species-specific) behaviors and safety. The second half of the chapter discusses fish welfare assessment, including the selection of welfare indicators, which can be both input-based and outcome-based. It also discusses issues related to conducting on-farm welfare assessment and how to integrate welfare outcomes into an overall welfare assessment. The final section of the chapter looks at some future developments and directions related to the validation and standardization of welfare indicators, precision fish farming and new sensor-based technologies aimed for tracking and monitoring the welfare of individual fish.