Available for the first time in English, this classic text by Jun Kondo describes the Kondo effect thoroughly and intuitively. Its clear and concise treatment makes this book of interest to graduate students and researchers in condensed matter physics. The first half of the book describes the rudiments of the theory of metals at a level that is accessible for undergraduate students. The second half discusses key developments in the Kondo problem, covering topics including magnetic impurities in metals, the resistance minimum phenomenon, infrared divergence in metals and scaling theory, including Wilson's renormalization group treatment and the exact solution by the Bethe ansatz. A new chapter has been added covering advances made since the Japanese edition was published, such as the quantum dot and heavy fermion systems.