'Learning' refers to change in knowledge behaviour as a function of experience. Fundamentally, learning is about making connections. The simplest forms of learning are habituation (which indicates that a stimulus is irrelevant and we can afford to stop noticing it) and sensitization (which indicates that a stimulus is relevant and threatening, so we *really* notice it).

While learning is distinct from instinct, which is unlearned and biologically hardwired, learning itself is based on making connections between stimuli in the environment and those instincts. We have automatic and instinctive ('unconditioned') responses to stimuli that are meaningful to us ('unconditioned stimuli'). For example, the automatic, instinctive, unconditioned response to food is salivation.

Classical conditioning is a simple form of learning by which we learn to respond to a neutral stimulus as if it were an unconditioned stimulus. Thus, the previously neutral stimulus becomes a 'conditioned stimulus'. This happens when the neutral stimulus reliably predicts the unconditioned stimulus. Responding to the conditioned (and previously neutral) stimulus as if it were the unconditioned stimulus functions to prepare us to encounter the unconditioned stimulus. In the famous example based on Ivan Pavlov's research, a dog comes to salivate (CR) to a tone (NS → CS) because the tone predicts the delivery of food (UCR).

Important applications of classical conditioning include advertising (brands are paired with stimuli that elicit positive emotions), treatment of phobias (the previously feared stimulus is associated with relaxation), and drug rehabilitation (the body's reaction to drugs is conditioned to the environment in which the drugs are used).