  When there is more than one source of light along the line of sight to a
gravitationally lensed object, the characteristics of the observed light curve
are influenced by the presence of the light that is not lensed. In this paper
we develop a formalism to quantify the associated effects. We find it useful to
introduce the concept of a ``blended Einstein radius" and an ``effective
Einstein radius", to describe the probability that a mass will serve as a lens,
or that a source will be lensed in an observable way. These considerations lead
to generic predictions about the results of gravitational microlensing
experiments. One example is that the optical depth for the lensing of giants is
greater than that for the lensing of main sequence stars; for any given
population of sources and lenses this effect can be quantified. We test and
sharpen these predictions by performing a series of Monte Carlo simulations. We
also outline general methods to (1) test whether specific events which fail to
be fit by point-mass light curves are viable candidates for blended events, (2)
use the effects of blending to learn more about the lensing event than would be
possible if there were no blending, and (3) include the effects of blending
when inferring properties of underlying populations through the statistical
study of lensing events.
