Molecular line emission in the mm- and submm-wave domains is found to be a very useful tool to study planetary nebulae (PNe). Molecular lines, particularly the low-$J$ transitions of carbon monoxide, are known to probe most of the nebular material in young PNe and protoplanetary nebulae (PPNe). Many quantitative results on these objects have been so obtained, including general structure, total mass and density distribution, kinetic temperatures, velocity fields, etc. In more evolved PNe, however, the molecular line intensity (or even detection) is quite uncertain, due to photodissociation. Molecular lines often trace just certain parts of these nebulae, like equatorial tori or axial clumps. Other molecules are in general more affected by photodissociation than CO, but photo- or shock-induced chemistry produces high abundances of radicals and large molecules in some intermediate-evolution objects. The presence of rotating disks in some objects is finally discussed.