Implementation science has evolved to use theoretical approaches to better understand and explain how and why implementation succeeds or fails. This article proposes a taxonomy of five categories of theoretical approaches: process models, determinant frameworks, classic theories, implementation theories, and evaluation frameworks. These categories help in selecting and applying appropriate approaches in implementation research and practice, and foster interdisciplinary dialogue. Process models describe and guide the process of translating research into practice, identifying phases or stages of the research-to-practice process. Determinant frameworks identify factors influencing implementation outcomes, such as barriers and enablers, but often lack detailed guidance. Classic theories and implementation theories explain the mechanisms behind implementation outcomes, while evaluation frameworks assess implementation outcomes like acceptability, adoption, and sustainability. Theoretical approaches in implementation science vary in their focus and application. Process models emphasize the sequence of implementation steps, while determinant frameworks consider multiple levels of influence. Classic theories and implementation theories provide explanations for implementation outcomes, and evaluation frameworks assess the effectiveness of implementation efforts. Despite overlaps between categories, each approach has distinct purposes and characteristics. Determinant frameworks often lack detailed guidance, while process models focus on the sequence of implementation. Classic theories and implementation theories explain the mechanisms behind implementation outcomes, and evaluation frameworks assess the effectiveness of implementation efforts. Implementation science is interdisciplinary, drawing from psychology, sociology, and organizational theory. Theoretical approaches are essential for understanding and improving implementation outcomes, but they must be applied thoughtfully to avoid oversimplification. While theories can provide valuable insights, they should be complemented by empirical research and practical experience to ensure effective implementation. The use of theoretical approaches in implementation science is increasingly recognized as important for reducing the research-practice gap and improving the application of evidence-based practices.Implementation science has evolved to use theoretical approaches to better understand and explain how and why implementation succeeds or fails. This article proposes a taxonomy of five categories of theoretical approaches: process models, determinant frameworks, classic theories, implementation theories, and evaluation frameworks. These categories help in selecting and applying appropriate approaches in implementation research and practice, and foster interdisciplinary dialogue. Process models describe and guide the process of translating research into practice, identifying phases or stages of the research-to-practice process. Determinant frameworks identify factors influencing implementation outcomes, such as barriers and enablers, but often lack detailed guidance. Classic theories and implementation theories explain the mechanisms behind implementation outcomes, while evaluation frameworks assess implementation outcomes like acceptability, adoption, and sustainability. Theoretical approaches in implementation science vary in their focus and application. Process models emphasize the sequence of implementation steps, while determinant frameworks consider multiple levels of influence. Classic theories and implementation theories provide explanations for implementation outcomes, and evaluation frameworks assess the effectiveness of implementation efforts. Despite overlaps between categories, each approach has distinct purposes and characteristics. Determinant frameworks often lack detailed guidance, while process models focus on the sequence of implementation. Classic theories and implementation theories explain the mechanisms behind implementation outcomes, and evaluation frameworks assess the effectiveness of implementation efforts. Implementation science is interdisciplinary, drawing from psychology, sociology, and organizational theory. Theoretical approaches are essential for understanding and improving implementation outcomes, but they must be applied thoughtfully to avoid oversimplification. While theories can provide valuable insights, they should be complemented by empirical research and practical experience to ensure effective implementation. The use of theoretical approaches in implementation science is increasingly recognized as important for reducing the research-practice gap and improving the application of evidence-based practices.