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The species of Niesslia including anamorphs formerly known as Monocillium form a distinct group in the Hypocreales and can be delimited by several unique morphological characters. Niesslia species inhabit decaying plant substrates, such as leaf litter or bark of diverse plants, especially coniferous hosts. Certain species occur on and may parasitize lichens, fruiting bodies of other fungi and even nematode eggs. Taxonomically, species of Niesslia are classified in the Niessliaceae. Niesslia species are recognised by their tiny superficial, mostly dark brown, shiny and typically spine covered ascomata. Monocillium-like phialides formed in situ and in culture are usually partly or entirely thick-walled. Phialides with wall thickening only in their basal parts sometimes expand in width to taper again into a conidiiferous neck, while others terminate in a non-sporulating vesicle. Based on the revised generic concept presented here, Niesslia accommodates 50 cultured species plus around 40 species known only from herbarium material. Some of the latter species may eventually have to be excluded from Niesslia but are maintained in the genus ad interim, until DNA sequences necessary for their accurate reclassification are available. The taxonomic concept based on morphological characters is corroborated by DNA sequences of phylogenetic marker loci such as the large subunit (28S) of the ribosomal RNA gene, internal transcribed spacer regions 1 and 2 including the 5.8S ribosomal RNA gene, and partial sequences of translation elongation factor 1-α, β-tubulin 2 and γ-actin genes. These DNA barcodes support most of the morphologically delimited species and the generic concept of Niesslia.