Magnetite is a naturally occurring iron-oxide mineral with applications in several industries. This monograph includes five chapters that detail various aspects of magnetite. Chapter One focuses on understanding magnetite and its modifications in the context of the cancer treatment modality magnetic hyperthermia. Chapter Two examines various aspects of synthesis and characterisations of magnetic nanoparticles, surface functionalization and enzyme immobilisation with improved catalysis, stability, and reusability. Chapter Three presents data on the effect of magnetoacoustic emission and its relation to the behaviour of the domain structure of magnetites formed under various conditions. Chapter Four highlights the detection of trace levels of pesticides and drugs in different environmental samples by using magnetite-molecularly imprinted polymers. Lastly, Chapter Five discusses the methods of synthesis and typical characteristics of magnetite-based nanocomposites that can be used for different applications.