This chapter considers various ways in which two or more lenses may be mounted together to form opto-mechanical subassemblies or assemblies. The treatment is by no means all inclusive. The examples were chosen to illustrate multiple uses of the techniques described in Chapter 4 for mounting individual lens elements and components. Included are discussions of means for establishing the proper spacing between elements, of lens assemblies with and without moving parts, of the lathe assembly technique, of typical refracting and reflecting microscope objectives, of binoculars and refracting telescopes, of assemblies involving plastic optical and mechanical parts, of techniques for coupling elements with liquids instead of cement, and of a variety of catadioptric instruments. Some proven techniques for aligning lenses in assemblies are also described.