ADP 1-01 guides Army professionals (both Soldiers and Army civilians) in their understanding of the entire body of professional knowledge and beliefs that shape the art and science of their profession. It addresses what doctrine is, why it is important, and which major ideas underlie it. The publication also discusses the most important taxonomies and terms used in the conduct of operations and the way they fit together as a single coherent whole. The principal audience for the ADP 1-01 is all members of the Army Profession. Commanders, staffs, and subordinates ensure that their decisions and actions comply with applicable United States, international, and in some cases host-nation laws and regulations. Doctrine is dynamic and changing based on lessons learned in current operations and training, adaptive enemies, and changes in force structure, technology, and social values. This publication provides the basic information necessary to understand Army doctrine and the ways it changes. It clarifies why various constructs exist and how they all fit together. It is a guide for professionals about the language of the profession. Starting with Baron von Steuben's Regulations for the Order and Discipline of the Troops of the United States, doctrine in various forms has guided the Army through peacetime and war. Lessons learned from world wars and other operations shaped and codified how Army forces operated. (Army forces refers to Army organizations whose role is to conduct operations in the field. The Army refers to the Army as an institution.) In the early 1900s, Army doctrine consisted of fewer than 40 field service regulations and drill manuals. Over time, doctrine grew to over 500 field manuals. Although they provided tactics and procedures, these publications lacked a clear hierarchy that served to both categorize and prioritize information. Leaders sometimes struggled to determine what was truly important for all professionals and what was important only to a branch or functional area. Additionally, as doctrine evolved, it saw a prolific growth of terms and expressions used. This growth sometimes obscured the relationship of terms and expressions such that Soldiers and their leaders did not clearly understand them. In 2011, the Army instituted Doctrine 2015 to bring clarity to doctrine. Doctrine 2015 restructures and reformats information. It identifies and distills overarching doctrine into Army doctrine publications (known as ADPs) and Army doctrine reference publications (known as ADRPs), while parsing details into field manuals (known as FMs) and Army techniques publications (known as ATPs); each publication type targets a more narrowly focused topic providing a clear hierarchy and reducing the number of publications. The need to quickly transition to Doctrine 2015 did not afford time for deeply examining some underlying issues. The Army repackaged existing doctrine emphasizing important information while deferring the larger question of why the information was important to Army forces. Soldiers and leaders must understand what Army doctrine is, what its purpose is, how it is organized, and why the information in doctrine is important. The precursor to this understanding is a definitive text on the why of Army doctrine-a doctrine primer. The doctrine primer becomes the standard for evaluating future doctrine, allowing the Army to discipline the establishment of terms and the categorization of operational knowledge. Chapter 1 first looks at what doctrine is and why it is important. Chapter 2 examines the doctrine landscape-the structure of doctrine, types of doctrine, the relationship of doctrine to concepts and lessons learned, and reasons for doctrine changes. Chapter 3 looks at the foundations of doctrine. Chapter 4 examines the terms and taxonomies of current doctrine. Finally, chapter 5 examines how the taxonomies work together to facilitate the conduct of operations.