Electrical and optical properties of crystalline semiconductors are important parts of pure physics and material science research. In addition, knowledge of parameters related to these properties, primarily for silicon and III–V semiconductors, has received a high priority in microelectronics and optoelectronics since the establishment of these industries. For control protocols, emphasis has recently been placed on novel optical measurement techniques, which have proved very promising as nondestructive and even non-contact methods. Earlier they required knowledge of the free-carrier-derived optical constants, related to the electrical conductivity at infrared frequencies, but interest in the optical constants of silicon in the visible, ultraviolet (UV) and soft-X-ray ranges has been revived since the critical dimensions in devices have become smaller. This chapter surveys the electrical (Sect. 21.2) and optical (Sect. 21.3) properties of crystalline silicon. Section 21.2 overviews the basic concepts. Though this section is bulky and its material is documented in textbooks, it seems worth including since the consideration here focuses primarily on silicon and is not spread over other semiconductors – this makes the present review self-contained. To avoid repeated citations we, in advance, refer the reader to stable courses on solid-state physics (e.g. [21.1,2]), semiconductor physics (e.g. [21.3]), semiconductor optics (e.g. [21.4]) and electronic devices (e.g. [21.5]); seminal papers are cited throughout Sect. 21.2. We realize how formidable our task is – publications on electrical and optical properties of silicon amount to a huge number of titles, most dating back to the 1980s and 1990s – so any review of this subject will inevitably be incomplete. Nevertheless, we hope that our work will serve as a useful shortcut into the silicon world for a wide audience of applied physics, electrical and optical engineering students.