Abstract: A Superconducting Quantum Interference Device (SQUID) was used for sensitive detection of nuclear spin precession at microtesla magnetic fields in thermally polarized liquids. Experimental studies were performed to demonstrate novel techniques and investigate new phenomena in nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) conducted in low static fields not, easily accessible in conventional systems. Coupled to an untuned superconducting input circuit, the niobium SQUID detected magnetic fields over a wide band down to ∼1 Hz with a field sensitivity of a few fT/Hz ½. A superconducting shield and an array of Josephson junctions protected the SQUID and the input coil from direct exposure to the external fields and large induced currents caused by such fields. A custom-built fiberglass cryostat or a Pyrex cryogenic insert provided thermal insulation between room temperature samples and the superconducting components in liquid helium. Following thermal polarization in fields of 0.5–300 mT, the magnetic field from 1H nuclear spins precessing in fields of 1–130 μT, corresponding to the Larmor frequencies of 40–5600 Hz, was detected by the SQUID with high sensitivity. This technique allowed a number of new experiments to be performed without a strong static field. The availability of variable fields for polarization relaxation allowed the measurement of the characteristic field dependence of the spin-lattice relaxation times in different materials over a wide range of fields; this was used to manipulate and enhance the image contrast in MRI of gel samples effectively. In NMR, the dynamics of nuclear spins in a rotating magnetic field were studied, and it was demonstrated that in the absence of any static field, nuclear spins in liquids can be polarized by a rotating field with frequencies much greater than the spin-lattice relaxation rate of the sample. Suggestions are made for future experiments that further address the unique opportunities as well as challenges presented by ultra-low field NMR and MRI.