Ed "Cass" Cassidy (born Chicago, May 4, 1923) is a drummer who was one of the founders of the rock group Spirit in 1967.

His family moved to Bakersfield, California, in 1931, and he started as a professional musician in 1937. He was in the Navy during the Second World War, and after his discharge worked at many jobs before becoming a fulltime musician again. At one time in the late 1940s he played 282 consecutive one-nighters in 17 states. He worked in show bands, Dixieland combos, country and western bands, and on film soundtracks, as well as having a brief stint with the San Francisco Opera.

In 1950 he enrolled at college to get a musical teaching credential, but after about a year decided to leave and move to southern California to meet more jazz musicians and perhaps form a group of his own. During this period he performed together with many leading jazz musicians including Art Pepper, Julian Cannonball Adderley, Roland Kirk, Lee Konitz and Gerry Mulligan.

With Taj Mahal and Ry Cooder, he formed the Rising Sons in 1964. After that, he formed the Red Roosters in 1965, with his young stepson Randy California, Jay Ferguson and Mark Andes. Adding John Locke, they became Spirit in 1967. A powerful, melodic and creative drummer, Cassidy was uniquely recognizable when on stage with Spirit. He sported a shaved head (very unusual at that time), always wore black, and instead of the double-bass drum kit that was popular at the time, he used a single large parade bass drum turned sideways, with pedals on each side. He had a proclivity for playing extended drum solos with his bare hands instead of drumsticks.

Cassidy has played with various line-ups of Spirit on almost 20 albums over almost 40 years. From the mid 1970s, Cassidy has also worked as an actor, including live improvisation and appearances on the TV series General Hospital and in movies. He also runs drum clinics, as well as lecturing and writing.