Hydrodynamical calculations of Planetary Nebulae (PNe) over 25000 yrs of evolution which include timedependent effects of ionization as well as variable central stars winds and parameters consistent with stellar model calculations show a great variety of velocity and density structures which strongly deviate from the often assumed “homogeneous shells, expanding with constant velocities” (Marten & Schönberner, 1991, hereafter MS). By means of a static photoionization code we calculate the surface brightness in the 10 most prominent nebular emission lines for density structures of the model sequence “VS” as given by MS. The obtained radial emissivities are used together with the velocity and temperature structure of the ionized gas to calculate (thermally broadened) line profiles in order to derive “measured” expansion velocities. We compare the theoretical surface brightnesses with observations and demonstrate some difficulties in the interpretation of nebular expansion velocities, expansion distances and ages.