Applied sciences in the 20th century have developed and used unitary transforms for concentrating energy. Now, the challenge lies in the expression and use of redundancy to build redundant systems. The Mojette transform is a very simple transform using only additions but with strong properties that break this challenge. The first part of the book gives the basics of the Mojette transform both mathematically and the corresponding optimal algorithms. The second part exemplifies its use through different fields: image representation, watermarking, medical imaging, distributed storage, information and cryptography. This book about a discrete exact Radon transform explains how to usefully produce and cope with redundancy for solving 21st century problems.