Interferometry and the study of protoplanetary disks

Interferometry offers access to scales that are unresolvable using single dish telescopes, which is especially important when investigating objects with small angular sizes, such as protoplanetary disks. This Chapter introduces the concept of interferometry, and describes the basic aspects of interferometric observations when considering long (i.e. sub-millimetre to radio) wavelength observations. Examples of recent work using both continuum and spectral line observations are investigated, and their diagnostic power is examined. Finally, a selection of currently available facilities are=ussed, and the prospects of future instruments are explored