  Low-mass planets have an extraordinarily diverse range of bulk compositions,
from primarily rocky worlds to those with deep gaseous atmospheres. As
techniques for measuring the masses of exoplanets are advancing the field
towards the terrestrial regime, from ultra-short orbital periods to Venus-like
distances, we identify the bounds on planet compositions, where sizes and
incident fluxes inform bulk planet properties. In some cases, measurement
precisions of planet masses and sizes are approaching the theoretical
uncertainties in planet models.
  An emerging picture explains aspects of the diversity of low-mass planets
although some problems remain; do extreme low density low-mass planets
challenge models of atmospheric mass loss? Are planet sizes strictly separated
by bulk composition? Why do some stellar characterizations differ between
observational techniques?
  As the TESS mission begins, low-mass exoplanets around the nearest stars will
soon be discovered and characterized with unprecedented precision, permitting
more detailed planetary modeling and atmospheric characterization of low mass
exoplanets than ever before.
