The miraculous powers of modern calculations are due to three inventions: Arabic Notation, Decimal Fractions and Logarithms (Cajori, 1919). However, while the first two of these inventions have been investigated in great detail by researchers in mathematics education, logarithms have received almost no attention. The mathematical concept of a logarithm plays a crucial role in many aspects of our existence, especially when dealing with incredibly large or incredibly small quantities. This book introduces a research that analyzes and describes the issues involved in high school students' understanding of logarithms and highlights the most common difficulties students face as they develop their understanding. An author offers a systemized description of students' difficulties with logarithms, as well as several explanations of the sources of these difficulties. Among the few implications for teaching practice that were developed in the presented study was the initial introduction of the concept. In the traditional curriculum the logarithm is introduced and defined as an exponent. However, historically logarithms were developed completely independently of exponents.