Photonic devices (e.g., optical fibers, 2D waveguides) are commonly used on telescopes worldwide and have been hugely successful in terms of their scientific impact. Coupling light efficiently into photonic devices is critical because most astronomical sources generate low photon count rates at the detector, even when coupled to large telescope apertures. Recent developments in adaptive optics have helped to improve the coupling to optical fibers, although efficient coupling to single-mode fibers remains a challenge. We appraise the theoretical limits to this coupling efficiency in both diffraction limited and seeing limited conditions, and we review methods to improve coupling efficiency with a view to recent advances in astrophotonics.