Nearly all of the narratives about understanding the composition of planets come from our understanding of the solar system. However, this is a terribly small sample size. The era of exoplanet characterization allows us to understand planets with a tremendously larger scope. In this talk I will cover several topics that shed light on planetary composition. In the realm of transiting planets, planetary structure models, compared to observations from Kepler and ground-based surveys, show our solar system's population of rocky planets is potentially unusual since they lack thick hydrogen atmospheres. However, our two gas giants, Jupiter and Saturn, fit well within the population of transiting giant exoplanets, in that all these planets show similar enrichments in "metals" compared to their parent star's composition. Next, we are obtaining spectra of transiting planet atmospheres with Hubble to constrain atmospheric molecular abundances, which also point toward metal-enrichment for gas giants. Finally, we are developing new theoretical tools to extract constraints on atmospheric abundances from the spectra of brown dwarfs and giant planets to look for composition clues that distinguish these two populations. As a whole these advances will yield fundamentally new insights on planetary formation and evolution.