  After introducing gauge-invariant cosmological perturbation theory we give an
improved set of governing equations for multiple fluids including energy
transfer. Having defined adiabatic and entropic perturbations we derive the
``conservation law'' for the curvature perturbation on large scales using only
the energy conservation equation. We then investigate the dynamics of assisted
inflation. By choosing an appropriate rotation in field space we can write down
explicitly the potential for the weighted mean field along the scaling solution
and for fields orthogonal to it. This allows us to present analytic solutions
describing homogeneous and inhomogeneous perturbations about the attractor
solution without resorting to slow-roll approximations. Finally we analyze the
simplest model of preheating analytically, and show that in linear perturbation
theory the effect of preheating on the amplitude of the curvature perturbation
on large scales is negligible. We end with some concluding remarks, possible
extensions and an outlook to future work.
