Superconducting Quantum Interference Devices (SQUIDs), consisting of two Josephson junctions in a closed superconducting loop, are exquisitely sensitive detectors of magnetic flux. In recent years, we have built magnetic resonance imaging (MRI) scanners based around these detectors which are capable of in vivo imaging at ultra-low (132 microTesla) fields, rather than the several Tesla of conventional MRI. I’ll discuss the challenges and unique advantages of ultra-low field MRI, including enhanced contrast between tissues types such as normal and cancerous prostate tissue which are nearly identical at high fields.