This teacher's guide contains nine topics in number theory. Suggested questions for the teacher, short investigations, and possible exercises for the student are included. Chapter 1 is an introduction to sequences and series using geoboard activities involving triangular numbers, square numbers, rectangular numbers, and pentagonal numbers. The second chapter concerns prime numbers and generating prime numbers; included is a computer program in BASIC to find all prime numbers less than any given number. Chapter 3 concerns divisors, with geometrical interpretation of the greatest common factor. The next chapter is on congruence of numbers in modular arithmetic with exercises leading to divisibility tests. A team game with rules which can be used for reviewing number theory is presented followed by a chapter on Pythagorean systems. The final chapters of the guide present a brief description of linear diophantine equations. (JW)