Patrick (Pat) Thaddeus, a founder of the field of astrochemistry, which blossomed in the late twentieth century, was largely responsible for recognizing that our galaxy is filled with a vast number of giant molecular clouds, the nurseries of forming stars. By combining his skills in laboratory spectroscopy and astronomical observation, Thaddeus and his associates discovered a significant fraction of the known interstellar molecules, including organic rings, long carbon chains, and negatively charged ions. His ability to identify previously undetermined spectral lines was remarkable. He built two identical 1.2-m radio telescopes, one for each hemisphere, with which he mapped the distribution of carbon monoxide in our galaxy, the Magellanic Clouds, and the nearby galaxy M31, over a period of 40 years. Early in his career, he modeled the atmosphere of Venus, to help understand its anomalous temperature profile. He also made an important early measurement of the cosmic microwave background (CMB) that confirmed its blackbody nature. Although he preferred to pursue his own research agenda with modest instruments, he guided the development of major new instrumentation in radio astronomy.