To avoid our animals developing fear responses towards the surroundings or us, we need to know the principles behind animal learning. Using the principles described in this chapter, it is also possible to adapt the environment and our own behavior to ensure — by determinedly focused handling, training, and management procedures — that the animal mostly will learn responses and behaviors that increase the welfare of each individual animal. A prerequisite for learning is the ability to perceive stimuli, both external and internal. An external stimulus may be a physical change such as a change in wavelength of light in the environment or a sound. An internal stimulus could be a change in heart rate or the experience of nausea. Learning is classified as either non-associative learning or associative learning. When the strengthening or weakening of a behavior is a function of the consequences of that behavior, the type of learning involved is referred to as operant learning.