Although filamentous fungi are used extensively for protein expression, their use for the production of heterologous glycoproteins is constrained by the types of N-glycan structures produced by filamentous fungi as compared to those naturally found on the glycoproteins. Attempts are underway to engineer the N-glycan synthetic pathways in filamentous fungi in order to produce fungal expression strains which can produce heterologous glycoproteins carrying specific N-glycan structures. To fully realize this goal, a detailed understanding of the genetic components of this pathway in filamentous fungi is required. In this review, we discuss the characterization of the $alpha$-mannosidase gene family in filamentous fungi and its implications for the elucidation of the N-glycan synthetic pathway.