We model the photoelectric emission from and charging of interstellar dust and obtain photoelectric gas heating efficiencies as a function of grain size and the relevant ambient conditions. We employ improved estimates for photoelectric thresholds, yields, and electron capture rates. Using realistic grain size distributions, we evaluate the net gas heating rate for various interstellar environments and find less heating for dense regions characterized by RV =5.5 than for diffuse regions with RV=3.1. We provide fitting functions that reproduce our numerical results for photoelectric heating and recombination cooling for a wide range of interstellar conditions. Finally, we investigate the potential importance of photoelectric heating in H II regions, including the warm ionized medium. We find that photoelectric heating could be comparable to or exceed heating due to photoionization of H for high ratios of the radiation intensity to the gas density. We also find that photoelectric heating by dust can account for the observed variation of temperature with distance from the Galactic midplane in the warm ionized medium.