  As an introductory lecture to the workshop an overview is given over
continuum models for homoepitaxial surface growth using partial differential
equations (PDEs). Their {\em heuristic derivation} makes use of inherent
symmetries in the physical process (mass conservation, crystal symmetry, ...)
which determines their {\em structure}. Two examples of applications are given,
one for large scale properties, one including crystal lattice discreteness.
These are: (i) a simplified model for {\em mound coarsening} and (ii) for the
transition from {\em layer-by-layer} to {\em rough growth}. Virtues and
shortcomings of this approach is discussed in a concluding section.
