The career of Jim Zimmerman, beginning with a solid foundation in electronics and cryogenics, reached a turning point in 1965 when he became coinventor of the rf SQUID (Superconducting QUantum Interference Device), while working at the Scientific Laboratory of the Ford Motor Company in Dearborn, Michigan. Recognizing the exquisite sensitivity of the SQUID as an amplifier and magnetometer, Zimmerman devoted the remainder of his career, at Ford and later at the National Bureau of Standards, to the further development of the SQUID and its applications. In 1969, Zimmerman also helped found SHE Corporation, which marketed the first commercially successful SQUID. While at NBS, Zimmerman introduced two variations, the SQUID gradiometer and the fractional-turn SQUID, to enhance the sensitivity of SQUIDs in special situations. He also developed an improved understanding of SQUID dynamics by exploring the pendulum analog using carefully made models, work that has benefited a generation of students. Putting the SQUID to work, Zimmerman investigated applications in metrology, biomagnetism, and geophysics. Notably, he participated in collaborations that recorded the first magnetocardiogram made with a SQUID and the first magnetoencephalogram of an evoked auditory response. Later, Zimmerman explored closed-cycle refrigeration as a means of making SQUIDs more useful outside the laboratory environment, and in 1977 he demonstrated an operating SQUID cooled to 8.5 K by a Stirling-cycle refrigerator made largely of plastic. Zimmerman is remembered for his keen physical insight, the elegance and simplicity of his experiments, and his willingness to question conventional wisdom in all aspects of life.