This book provides readers with an accessible and up-to-date introduction to the field of low-field MRI, which is currently seeing a resurgence in both research and commercial activity. It begins by presenting a historical overview of MRI system design and discussing current developments. It then analyzes the underlying physics of MRI from a semi-classical perspective before describing the major hardware components of low-field scanners (including the magnet, coils, transmitters, receivers, gradient systems, and digital processors) in detail. Several examples of each component are described to solidify the reader's understanding of the major challenges and trade-offs involved in designing these complex devices. Finally, the issues involved in integrating these components within a working system are highlighted by presenting the architecture, design, and test results of two fully functional low-field MRI scanners that were designed and developed by the authors.