Domestic animals are animals whose living conditions and reproduction, among other things, are controlled by man. As such, the current discussion about the welfare of domestic animals is similar for farm, companion, laboratory, and zoo animals. It concerns identification of the behavioral and physiological needs of the animals and development of living conditions that enable them to satisfy these needs. The paper describes two approaches that have been used in behavior biology to identify such needs. One approach is the measurement of stress responses that may be activated when an animal`s needs are not fulfilled. The other approach is the use of operant conditioning techniques to establish demand functions by which the motivation of an animal to perform a specific behavior is measured. It is concluded that, since welfare is characterized by the absence of a number of factors, such as stress, pain, fear, disease, hunger etc., many types of measurements must be applied to ensure optimal welfare.