This book aims to provide the reader a neuroscientific understanding surrounding a very simple question: how do we learn not to fear? Exploring answers to this question is very important for two reasons. First, learning about the neural mechanisms of fear extinction is of relevance to everyone's life - it is such a basic yet relevant question to our daily experiences. Therefore, understanding brain mechanisms of fear and its regulation is essential from a basic neuroscience point of view. Second, excessive fear and the inability to regulate its expression is one of the hallmarks of fear-, anxiety-, trauma-, and stressor-related psychopathologies. And as such, learning about how fear is acquired, stored, expressed, and regulated could help advance our understanding of the etiology of psychopathology, the maintenance of symptoms pertaining to failure to regulate fear, and could help us develop novel therapeutics to equip providers and patients with the tools to better quell their fears. The contributions contained in this book are provided by experts in the fields of basic and clinical neuroscience, experimental and clinical psychology, and neuropsychiatry. The contributions are organized to start the reader with basic definitions of how we define fear, how we study its neural circuits at the molecular and cellular levels, how to study human behavior and the brain using state-of-the art experimental and statistical tools, to how much fear contributes to psychopathology. This volume ends with current advances aimed to enhance the capacity to extinguish fear; a clinical result that would aid in the treatment of multiple psychiatric disorders, followed by a discussion on future directions of this highly important and relevant field.