Development of a knowledge base is an iterative and ongoing process that requires periodic analysis and synthesis of an entire body of work. This article examines 4 related levels of theoretical thinking that are currently used in developing knowledge for nursing practice, education, and science: meta-theory, grand theory, middle-range theory, and micro-range theory. Each level of theory is discussed according to typology, scope, and generalizability, level of abstraction, and role. Suggestions are made for clarification of terminology, and examples are provided for each level of theoretical thinking. Evidence associated with the 4 levels of theoretical thinking is discussed, and applications for use of the levels of theoretical thinking to meet future challenges in nursing's knowledge development are offered.