Filamentous fungi are lower eukaryotes increasingly used for expression of foreign proteins ranging from industrial enzymes originating from other fungi and bacteria to proteins of mammalian origin, such as antibodies and growth factors. Their strengths include an excellent capacity for protein secretion and their eukaryotic protein processing machinery. Proteins secreted from filamentous fungi are modified in the secretory pathway, with folding, proteolytic processing, and addition of glycans being the main modifications. Unlike from many other expression systems, however, plasmids and host strains for expression of gene products in filamentous fungi are not readily available commercially, and the expression system must thus be stitched together in the laboratory. In this overview, the key elements of fungal expression systems are discussed from a practical point of view and with a view towards the future. The principles and considerations presented here can be applied to a range of filamentous fungi. © 2018 by John Wiley & Sons, Inc.