  The properties of interstellar grains can now be defined by a rapidly growing
wealth of observational data. We rely upon models to combine these data with
unobserved properties such as the size distribution of grains, their structure
and shape, and their detailed chemical composition, in a self-consistent
manner. I will review the observational constraints which define the boundaries
for possible models and will then summarize the features of the three
quasi-comprehensive grain models most commonly referred to in the current
literature. I will conclude by discussing recent work dealing with single
aspects of interstellar grains and the phenomena attributed to them.
