I discuss an instructional model that I have used in my number theory classes. Facets of the model include using small group work and whole class discussion, having students generate examples and counterexamples, and giving students the opportunity to write proofs and make conjectures in class. The model is designed to actively engage students in the classroom, develop conceptual understanding, and improve students' proof-writing abilities. Classroom activities for implementing the model are offered for the concepts of divisibility, congruence, and Euler's phi-function.