Describes the controversy over the merits of behavioral vs physiological indicators of animal welfare. The debate is illustrated with behavioral and physiological responses of pigs. Pigs respond to stress with an increase in adrenocortical activity; aggression; and abnormal behaviors such as bar biting, sham chewing, and head weaving, called stereotypies (STs). Causes and functions of STs are unclear, but STs may be regarded as a mechanism that helps animals cope with environmental change. Data from research on housing systems for pigs and the human–animal relationship will help in the evaluation of animal welfare. (PsycINFO Database Record (c) 2012 APA, all rights reserved)