The third edition of Foundations of Crystallography with Computer Applications is a textbook for undergraduate and graduate students studying the solid state in chemistry, physics, materials science, geological sciences, and engineering. It takes a straightforward, logical approach to explaining how atoms are arranged in crystals and how crystal systems are related to each other. New to this edition is the inclusion of interactive Starter Programs in Python, which allow the students to focus on concepts and not treat crystallographic programs as "black boxes." Since many students have trouble visualizing three dimensional constructions, this book begins with detailed discussions in two dimensions leading up to the three-dimensional understanding. The first seven chapters introduce the fundamental principles, Chapter 8 suggests student projects, and the final seven chapters give detailed examples of the seven crystal systems. Key features: Uses Python, the leading open-source scientific language, with libraries including NumPy for matrix manipulations, Matplotlib for graphics, and Mplot3d for interactive 3-D modeling. Provides a gentle introduction to Python with Jupyter Notebooks, which combine interactive code and formatted documentation. Color codes both point group and space group diagrams using a new scheme devised by the author to emphasize the change of handedness of the symmetry operations and their consequences. Suggests student projects with data that can be found in the free Teaching Subset of the Cambridge Structural Database, the American Mineralogist Crystal Structure Database, and others. This book’s thorough but accessible style gives students a strong foundation in the subject. Over one thousand students have successfully used this book at Virginia Tech, and many more will continue to benefit from this new edition.