The scientific study of the Biology of behvior is called as “Biopsychology”. Behavior is aroused by some change in the conditions. Such changes are commonly called “stimuli” or “stimulus”. A stimulus is then anything that makes an organism do something and what the organism does is known as a response. Human behavior then consists of responses to stimuli. Different stimuli evoke different responses. Many have tried to understand behaviors by analyzing them into stimulus response combinations. These stimulus response combinations can be very simple as very compiled. For example, when hand touch matchstick give we automatically withdraw own hand. Scene, he writes the story of the events or scene. Sometimes stimulus response combinations are innate also. This is the case with reflex actions most of these S-R combinations in the adults are the request of learning and experience. A stimulus can be external or internal. External stimulus are a sound, a pinprick, mobile sound, the sight of a person, etc. Internal stimulus includes some chemical changes inside the body, physiological changes, etc. And sometimes sudden memories of the past much also act on stimuli. So stimulus is anything that makes an organism to act. Thus, a stimulus can in defined as any object, events or experiences that causes a response, the reaction of an organism. Responses can be external or overt and response can be internal or covert. Overt responses are directly observable while covert responses are not observable directly. Again responses can see immediate or responses can be delayed, i.e. the response are also innate or acquired. Normal life is possible only because of the fact that certain stimuli are associated with certain response due to innate nature or due to the process of learning.