  We review some recent results of the theory of Lie systems in order to apply
such results to study Ermakov systems. The fundamental properties of Ermakov
systems, i.e. their superposition rules, the Lewis-Ermakov invariants, etc.,
are found from this new perspective. We also obtain new results, such as a new
superposition rule for the Pinney equation in terms of three solutions of a
related Riccati equation.
