While optical and quantum efficiency are on the rise, and spectrographs becoming massively multiplexed, measuring spectral energy distributions of astronomical sources with accuracy remains a challenge. In addition to atmospheric refraction, extinction, variability, and limited apertures of instrument entrance slits and optical fibers, accurate calibration is hampered by issues such as a limited choice of reliable standard stars. Modeling stellar spectral energy distributions has seen good progress, but some weaknesses survive, especially for late-type stars. This article provides an overview of these matters and discusses observation, calibration, and modeling strategies for future large spectroscopic surveys.