  I review the standard analysis of adiabatic scalar and tensor perturbations
produced by slow-roll inflation driven by a single scalar field, before going
on to discuss recent work on the role of non-adiabatic modes during and after
inflation. Isocurvature perturbations correlated with adiabatic modes may be
produced during multi-field inflation and would give valuable information about
the physics of the early universe. A significant contribution from correlated
isocurvature perturbations is not ruled out by present data, but should be
either detected or ruled out by future observations.
