This elementary number theory fascinatingly bridges the gap between school and university mathematics. Starting from the essential tools of mathematics, namely mathematical reasoning and proof, it addresses exciting and easily understandable questions about prime numbers and other types of numbers, as well as their applications in cryptography and ISBN codes. Highlights of the book include the proof of Fermat's conjecture for the special case n=4 and construction problems using compass and straightedge. Detailed and entertaining explanations, historical backgrounds, and insights into advanced mathematics such as linear algebra, analysis, and geometry effortlessly pave the way for a deeper engagement with mathematics. Numerous exercises, some with complete solutions, along with 100 illustrations, complete the presentation.