Bohr's first acquaintance with the subject of penetration of charged particles through matter was as early as in 1912 when he treated the absorption of &agr; and &bgr; rays on the basis of Rutherford's atomic model. From then on he kept a lifelong interest in the subject, often using it as an important test of the methods of atomic mechanics. His last paper on penetration, written together with Jens Lindhard, dealt with electron capture and loss and was published in 1954. Part I of this volume follows Bohr's work on penetration theory based on classical mechanics. Part II deals with the general theory of penetration, taking quantum-mechanical considerations into account.