We discuss the many factors affecting the reliability of GaAs HBTs that we have encountered starting from the early days of AlGaAs-emitter HBTs through the present day use of InGaP-emitter HBTs. We discuss both wearout and infancy failure modes and try to distinguish fundamental (i.e., unavoidable) from nonfundamental failure modes. We have found that infant failures are dominated by substrate dislocation density, which can limit long-term-reliable circuit sizes to under ~1000 transistors.