Endocrine feedback system may not be appreciated by conventional hormone level estimation at a single point of time. Our body also needs to be looked at as a dynamical system, a dissipative structure. Specifically, the information processing at the neuroendocrine level must be guided by a science that is not random but deterministic. For sustenance of life body environ must always (at the present moment) balance itself delicately and relentlessly. In this dynamical system, feedback control will essentially be mathematical control that has to be understood in the light of physical control theory. Whatever goes around inside the body must be originating from chemical reactions, but factors controlling the nature and rate and limit of those reactions are multi-relation oriented. The outcome of such relationship is not limited to linear reactions but dependent on some "non-material entity", like balancing while cycling or walking on a rope. This non-material entity is described as "feedback" relationship. This book tries to elucidate how and why such kind of relationship must be understood in order to know this reportedly simple subject of endocrine feedback.