  A number of trends among the properties of exoplanets have become evident in
the years since the first one was announced in 1995. One particularly
interesting trend began to emerge in 1997 -- the incidence of giant planets
correlates with the metallicity of the host star. This has since been
established with a high degree of statistical significance by several research
groups. Other, more subtle, trends are beginning to appear as the sample size
continues to grow and the statistics improve. I review the state of our
knowledge concerning the observed compositional trends and their possible
causes and suggest several research directions.
