Recent advances in photoemission are allowing detailed studies of the role of collective many-body excitations in the decay of a photohole. The collective excitations include phonons, charge density waves and magnetic or spin excitations. With these developments angle resolved photoemission with its momentum resolving capabilities has become a powerful probe of the transport properties in condensed matter systems. We review these advances and examine the application of photoemission to studies of both metallic systems and strongly-correlated materials. The latter exhibit a rich variety of phenomena including high Tc superconductivity. In particular, we examine the nature of the low-energy excitations and the role of dimensionality.