Associative and behavior systems accounts of Pavlovian conditioning ha
ve different emphases. The traditional associative account has focused
on the role of the unconditional stimulus (US) in strengthening stimu
lus associations according to a set of general laws. The behavior syst
ems account has focused on the relation of conditional responding to t
he preorganized perceptual, motor, and motivational organization engag
ed by the US. Knowledge of a behavior system enables successful predic
tion of the form and ease of conditioning as a function of the type of
conditional stimulus (CS), US, and the CS-US relation. At the same ti
me, Pavlovian manipulations act as a window on how a behavior system w
orks. Both associative and behavior systems accounts can be criticized
as incomplete and idiosyncratic. A comprehensive account of Pavlovian
conditioning could profit from their integration.