  This tutorial gives a general introduction to optical and infrared
interferometry, specifically addressing two questions: `Can I use VLTI to
observe my favourite object?' and `What will it tell me?' The observables
measured by an interferometer are fringe visibility and closure phase, which
can then be fitted by a model or used to reconstruct an image. In order to
measure fringes, the path lengths from the source to the detector via the
different individual apertures must be equalized. This requires some form of
fringe tracking, which constrains the type of observations that are feasible
and has important implications for the complexity and size of the resulting
image.
