In spite of the predilections of the authors, the work of Hanbury Brown and Twiss on intensity interferometry may have had a more powerful impact on quantum optics than on astronomy. That it could have such an impact on quantum optics is even more surprising when one considers that the their results are more easily explained in the context of classical optics than quantum optics. Nevertheless for more than 50 years, quantum optics has been inspired by intensity interferometry and continues to add new variations and insights. I will discuss experiments using cold atoms, both bosons and fermions, and from sources both coherent and incoherent. The observations yield useful information concerning what happens in atomic systems both when they interact with each other and when they interact with light.