 Dear students, in this module we shall discuss some basic principles that are central in the knowledge of physiology. A physiologist explores the physiological processes that are common to all animals. He also compares the effect of selective forces in developing these characters and these mechanisms during the process of evolution. This comparative approach provides us an insight about the patterns of physiological evolution and the adaptive value of these processes. This comparative and evolutionary approach has central place in physiology and because of this, we see that many basic concepts have been developed or emerged that apply to various physiological phenomena. We will discuss five basic themes that are essential for understanding the physiological processes. The first important concept of structure-function relationships is that functions are based on structures and various structures have evolved to fulfill certain functional requirements of animals. Dear students, another major concept in physiology and overall in biology is the concept of adaptation. Due to adaptation, the physiology of all animals of a species becomes very well matched to the environment. Related to the phenomena of adaptation, there are two more related phenomena. One is acclimatization and other is acclimation. Both these phenomena show adaptations but involving only one or two members of the species. Dear students, physiology ka ek aur bhunya di concept principle of homeostasis hai. This principle explains the tendency of an organism to maintain a relatively stable internal environment. One more basic concept in physiology is of feedback control systems. Feedback control systems provide a mechanism of self-regulation of physiological and biological processes, conformity or regulation. These are actually the responses that an animal gives to the changing environment, whether it conforms with its environment or it brings changes within itself, which is called regulation.