The emission of light, noninteracting particles, such as axions and Majorons, modifies the structure and evolution of stars. We show that the main effects of such an energy loss are to raise the central temperature and luminosity of stars and to reduce their lifetimes. We clarify previous discussions of stellar axion bounds by directly relating the effects of axions to observable stellar parameters. The concordance of the standard model of the Sun with observations yields a self-consistent bound on the coupling of light pseudoscalars to electrons: ${g}_{\ensuremath{\varphi}e}$5.4\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$. This corresponds to a lower limit on the Peccei-Quinn scale, (F/2${X}_{e}^{\mathcal{'}}$)\ensuremath{\ge}${10}^{7}$ GeV, comparable to the usually quoted solar bound. The lifetime of helium-burning stars can be used to place a very stringent and confident bound on the axion scale, (F/2${X}_{e}^{\mathcal{'}}$)\ensuremath{\ge}5.2\ifmmode\times\else\texttimes\fi{}${10}^{8}$ GeV, corresponding to ${g}_{\ensuremath{\varphi}e}$9.3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}13}$. We also discuss the consequences of axion emission for the solar-neutrino problem, for the ages of globular clusters, and for terrestrial solar axion detectors. Since the axion luminosity scales as a lower power of temperature than the nuclear-energy-generation rate, axion emission has a stronger influence on cool stars. We discuss the resultant changes in the mass-luminosity relation for low-mass stars and the effects on the low-mass cutoff for main-sequence stars.