The structure of protoplanetary disks is very complex and only partially understood. The composition of a disk depends strongly on age and other outside influences, e.g. the amount of matter available or nearby luminous O stars that blow away disk matter. Axial asymmetries can be present which make an analysis of such a disk even more difficult. In the following description of the disk structure, a symmetric and "typical" representation of a disk is used. Schematic plots of the dust and gas structure of a disk are shown in Fig. 1 .7.