Photoelectron and electron energy loss spectroscopies have been highly instrumental in revealing the various facets of electronic properties of materials. For a direct insight into two-particle correlations, a technique is needed that resolves two emitted electrons in coincidence. Herein, an overview on the experimental realization of correlation spectroscopy and the interpretation of the recorded spectra from theory is provided. The relation of the measured spectra to the details of the spin-, energy- and wavevector-resolved electron-electron interactions is focused upon. To disentangle the contributions of exchange from the charge-density correlation, positrons instead of electrons are used as projectiles. The short intrinsic time of the Auger decay and the neutralization of He²⁺ ions near a surface are used to estimate the characteristic timescale for the correlated electron dynamics in metals to be 40-400 attoseconds. The potential of conducting double photoemission studies with pulsed laser-based light sources is addressed.