This book aims to address cutting-edge progress in the area of synthesis and biomedical applications of magnetic nanomaterials. It compiles a broad spectrum from fundamental principles to technological advances, from synthesis and modification to biomedical applications along with biocompatibility. The main topics include principles in nanomagnetism, technologies for magnetic nanomaterials fabrication, developments in their biomedical applications, and the challenges in the toxicity in clinical translation. The first part introduced the principles of nanomagnetism and specific properties in magnetic nanomaterials. Then, some typical fabrication strategies in magnetic nanomaterials for controlled composition, morphologies and sizes are reviewed; and surface modification methods with better hydrophilcity and biocompatibility are presented. Next, magnetic nanomaterials-based applications in biomedical field are highlighted in detail, mainly including magnetic resonance image, magnetic hyperthermia, cancer therapy, multi-mode imaging, imaging-guided therapy and manipulate biological objects. Finally, biocompatibility issues caused by magnetic nanomaterials are also overviewed.