  Optical fibers have altered astronomical instrument design by allowing for a
complex, often large instrument to be mounted in a remote and stable location
with respect to the telescope. The fibers also enable the possibility to
rearrange the signal from a focal plane to form a psuedo-slit at the entrance
to a spectrograph, optimizing the detector usage and enabling the study of
hundreds of thousands of stars or galaxies simultaneously. Multi-core fibers in
particular offer several favorable properties with respect to traditional
fibers: 1) the separation between single-mode cores is greatly reduced and
highly regular with respect to free standing fibers, 2) they offer a monolithic
package with multi-fiber capabilities and 3) they operate at the diffraction
limit. These properties have enabled the realization of single component
photonic lanterns, highly simplified fiber Bragg gratings, and advanced fiber
mode scramblers. In addition, the precise grid of cores has enabled the design
of efficient single-mode fiber integral field units for spectroscopy. In this
paper, we provide an overview of the broad range of applications enabled by
multi-core fiber technology in astronomy and outline future areas of
development.
