The SQUID (Superconducting QUantum Interference Device) is an ultrasensitive detector of magnetic flux that combines the phenomena of Josephson tunneling and flux quantization.  With the aid of a superconducting flux transformer, the SQUID achieves a magnetic field noise below 10-15 THz-1/2.  Various applications of SQUIDs are briefly reviewed.  We have made an amplifier, based on a SQUID coupled to a resonant input circuit, which achieves a noise temperature within 20% of the quantum limit at 0.8 GHz.  This amplifier has been installed in the axion detector at Lawrence Livermore National Laboratory, and is expected to increase the frequency scan rate by three orders of magnitude.  We use a SQUID gradiometer, together with prepolarization of the nuclear spins, to obtain magnetic resonance images (MRI) in a magnetic field of 132 microtesla, corresponding to a proton Larmor frequency of 5.6 kHz—four orders of magnitude lower than in conventional MRI.  The spatial resolution is typically 1 mm.  The combination of prepolarization, spin relaxation in an adjustable field and detection in a microtesla field enables us to obtain relaxation time (T1)-weighted contrast images in fields ranging from 1 microtesla to 0.3 tesla.  Model systems consisting of different concentrations of agarose gel in water exhibit considerably higher T1-contrast in low magnetic fields than in high fields.  In preliminary experiments, we have measured T1 in ex vivo specimens of surgically removed healthy and malignant prostate tissue. At 132 microtesla, the values of T1 in healthy tissue are typically 60% higher than in malignant tissue, suggesting that ultralow field MRI may have clinical applications.