Driven by rapid progress in microelectronics and thin film technologies, magnetic sensors development continues to expand. This paper presents issues related to the principles, categorisation and applications of magnetic sensors. Special attention is paid to two types of sensors and their present and future impact: superconducting quantum interference devices (SQUID) magnetometers with their unsurpassed sensitivity, and giant magnetoresistance (GMR)/giant magnetoimpedance (GMI) based sensors as the most promising technology. A review of recent developments in SQUID technology and a discussion of limitations and aspects that could contribute to the wider acceptance of this technology, are presented. The giant magnetoresistance and giant magnetoimpedance effects have already found applications in magnetic sensing and have promise in other applications. Their unique characteristics and miniaturisation potential have contributed to the rapid acceptance of these technologies. The article describes the principles of the GMR and GMI effects along with recent developments in these technologies particularly in manufacturing techniques and materials. (C) 2003 Elsevier B.V. All rights reserved.