This book is about Information Retrieval (IR), particularly Classical Information Retrieval (CIR). It looks at these topics through their mathematical roots. The mathematical bases of CIR are briefly reviewed, followed by the most important and interesting models of CIR, including Boolean, Vector Space, and Probabilistic. The primary goal of book is to create a context for understanding the principles of CIR by discussing its mathematical bases. This book can be helpful for LIS students who are studying IR but have no knowledge of mathematics. Weakness in math impairs the ability to understand current issues in IR. While LIS students are the main target of this book, it may be of interest to computer science and communications students as well.