Conditioning is a learning process in which one’s behaviour becomes dependent on the occurrence of a stimulus in that environment. Two types of conditioning will be considered. In classical conditioning, an unconditioned stimulus such as meat will make a dog to salivate. The salivation is an unconditioned response. A neutral stimulus such as a bell will not produce any salivation in a dog. However, if a previously neutral stimulus such as a bell is paired with meat (an unconditioned stimulus), the bell becomes a conditioned stimulus and at the sound of the bell alone, the dog elicits a conditioned response by salivating. In contrast, operant conditioning does not depend on a preceding stimulus. It rather depends on what follows as a consequence of behaviour. Operant conditioning involves a two-way process: action (or behavior) operates on the environment while the environment at the same time shapes behaviour. Since behaviour is shaped efficiently through positive reinforcement, it follows that if behaviour results in a negative consequence, there is a decrease in that behaviour. The proponents of classical and operant conditioning are Ivan Pavlov (1849–1936) and Burrhus Frederic Skinner (1904–1990) respectively. This chapter explores the principles underlying classical and operant conditioning. These two types of conditioning are constantly used as a basis for evidence to support the understanding and treatment of psychological disorders. Not only that, different types of diagnosis and treatment are being developed and improved with the knowledge of how classical and operant conditioning works. Also, it is important to note that in real life, learning can be a mixture of both. For example, although someone may be classically conditioned to dislike a certain food, through operant conditioning, they might also be trained to seek out a preferred food. Also discussed is how each theory may be applied in furtherance of science teaching and learning.