Many people know Passive Inductive Transponder (PIT) tags as the microchips used to identify pets. These tags are also valuable fisheries research tools that can be used to identify and track individual fish. FISHBIO has conducted various studies using PIT tags since 2003, including tracking the movements of threatened California steelhead within their critical habitat. We surgically implant fish with small, lightweight PIT tags and set up PIT tag readers with antennas that stretch across a river channel. When a tagged fish swims by, the antenna’s magnetic field activates the tag’s microchip, which sends out a unique code identifying the fish. PIT tags do not require batteries, so they last for the entire life of a fish. We use PIT tags to study fish habitat use, migration timing, survival, and fish response to altered river flow and temperature.