Fifty years ago, R. Hanbury Brown and R.Q. Twiss, invented a new method to measure the angular diameter of stars, based on the observation of correlations in light. The analysis of their experiment in term of photons prompted hot discussions, and eventually led to the development of modern quantum optics, based on photon-photon correlation experiments. Similar quantum correlations can be observed with bosonic and fermionic atoms. I will present such experiments, after recalling the significance of the HBT landmark experiment.