The ACR Commission for Women and General Diversity is committed to identifying barriers to a diverse physician workforce in radiology and radiation oncology (RRO), and to offering policy recommendations to overcome these barriers. In Part 1 of a 2-part position article from the commission, diversity as a concept and its dimensions of personality, character, ethnicity, biology, biography, and organization are introduced. Terms commonly used to describe diverse individuals and groups are reviewed. The history of diversity and inclusion in US society and health care are addressed. The post-Civil Rights Era evolution of diversity in medicine is delineated: Diversity 1.0, with basic awareness, nondiscrimination, and recruitment; Diversity 2.0, with appreciation of the value of diversity but inclusion as peripheral or in opposition to other goals; and Diversity 3.0, which integrates diversity and inclusion into core missions of organizations and their leadership, and leverages its potential for innovation and contribution. The current states of diversity and inclusion in RRO are reviewed in regard to gender, race, ethnicity, sexual orientation, and gender identity. The lack of representation and unchanged demographics in these fields relative to other medical specialties are explored. The business case for diversity is discussed, with examples of successful models and potential application to the health care industry in general and to RRO. The moral, ethical, and public health imperative for diversity is also highlighted.