This is a concept paper in education specifically about curriculum theory. Curriculum theory is a sub-theory of educational theory. Theories provide views about the world. They explain reality. The process of education in general and the field of curriculum in particular are embedded in theory. Teachers and other educators rely on theory in research about the school curriculum. Curriculum theory gives direction and guidance in the process of curriculum planning, development, implementation, supervision, evaluation among others. This concept paper delves on curriculum theory: Its definition, development, functions and characteristics. The paper also explains categories of curriculum theories and theorists. The issues discussed in this paper can trigger research in theory development and inform educationists and teachers in theory development and research in curriculum.