An agonist is a chemical that activates a receptor to produce a biological response. Receptors are cellular proteins whose activation causes the cell to modify what it is currently doing. In contrast, an antagonist blocks the action of the agonist, while an inverse agonist causes an action opposite to that of the agonist. Receptors can be activated by either endogenous agonists (such as hormones and neurotransmitters) or exogenous agonists (such as drugs), resulting in a biological response. A physiological agonist is a substance that creates the same bodily responses but does not bind to the same receptor.

In the present book, ten typical literatures about receptor agonists published on international authoritative journals were selected to introduce the worldwide newest progress, which contains reviews or original researches on receptor agonists. We hope this book can demonstrate advances in receptor agonists as well as give references to the researchers, students and other related people.