Spatial division multiplexing attracts lots of attention for tackling the “capacity crunch” anticipated in the near future, and therefore various types of optical fibers and multiplexing methods have been researched intensively in recent years. This chapter introduces multi-core fibers for spatial division multiplexed transmission. It describes various characteristics specific to multi-core fibers, which have been elucidated theoretically and experimentally in recent years. Though there are many important factors, many pages are devoted especially to the description of inter-core crosstalk, which is crucial when signals are transmitted over each core independently. The chapter also describes other characteristics related to the improvement of the core density.