An introductory course to number theory. It roughly follows William Stein's public access textbook chapters 1-4, and supplementary materials and lessons will be provided, especially at the beginning of the semester, according to students' mathematical backgrounds. Topics discussed in this course include: modular arithmetic, prime factorization, cyclic groups and their subgroups, Euler's Phi function, the Chinese Remainder Theorem, prime testing, public key cryptography, and quadratic reciprocity.