This is an introduction to geometric algebra in n-dimensional Euclidean space and its application to manifolds and to calculus on manifolds. The treatment is moderately rigorous and is suitable for advanced undergraduates and beginning graduate students in mathematics though it should also be accessible to well-prepared students in physics, engineering, computer science, statistics, etc. Preparation in linear algebra and multivariable analysis as encountered in calculus as well as a modest amount of mathematical maturity should be sufficient.