Abstract: Magnetism is a fundamental physical phenomena that has played a critical role in the development of modern physics. Exploring the magnetic properties of materials has also been central to the development of modern information technology from data storage to computing itself. Magnetic property of a material is classically characterized by a quantity called magnetic susceptibility. The field perturbation induced by magnetic susceptibility can be measured by mechanical force balance or by SQUID. However, there is no universal methods for mapping the distribution of magnetic susceptibility within a large heterogeneous object such as the human body, as measuring the field outside an object cannot uniquely define the sources within. I will describe some recent developments in Magnetic Resonance Imaging (MRI) that have allowed the quantification of magnetic susceptibility within biological tissues non-invasively. I will also discuss its developing applications in biology and medicine and its current limitations.