Book excerpt: This book contains most updated information on synthesis of magnetic nanohybrids, their physio-chemical properties, and key biological applications. It highlights the complexity of nanoheterostructures, especially magnetic metal oxides, ferrites and doped magnetic nanomaterials, and discusses their potential applications in the early detection, imaging and treatment of cancer. It also covers the toxicity and risk assessment of multifunctional nanomaterials. Providing an overview of magnetic nanoheterostructures, it appeals to a wide audience, from beginners and graduate-level students to experts in academia and industry.