This volume tells personal and scholarly stories about the exemplary career and comprehensive impact of Professor Donald Baer, an internationally renowned behavioral scientist and the intellectual leader of applied behavior analysis. Throughout his distinguished career, Baer taught and modeled an experimental approach to understanding human behavior. His conceptual and scientific contributions impacted diverse fields, including child development, education, language, developmental disabilities, and clinical practice. In this book, his students and colleagues, many of whom are accomplished leaders in their respective fields, provide historical and conceptual accounts of Baer's influence based on their personal experiences of his mentorship. Samples of Baer's eloquence in written and oral discourse capture why he earned the moniker 'the Word Merchant.'