Obligately lichenicolous, non-lichenized ascomycetes which comprise le- cideoid taxa among their hosts are defined as "lecideicolous". Lecideicolous ascomycetes have been found on more than one hundred lecideoid lichen species belonging to thirty two - mostly lecanora- lean - genera. Systematically, lecideicolous ascomycetes are members of the following orders: Arthoniales, Dothideales, Helotiales, Lecanorales, Opegraphales, Ostropales, Sordariales, Yerrucariales. Forty-five species and two varieties are accepted. They are referred to twenty-one genera of which more than a half represent exclusively lichenicolous lines. Some lecideicolous ascomycetes are closely related to lichens, some to primarily non-lichenized groups of ascomycetes. Most lecideicolous ascomycetes probably receive their nutrients from the photobiont of the lichen association without damaging it. They are regarded as biotrophic parasites (or sometimes as parasymbionts). There are relatively few truly pathogenic parasites, and only a few obligately lichenicolous saprophytes. Only a few species are able to induce gall-like deformations of the host thalli or apothecia. Concerning the location of their ascomata, most lecideicolous species are specialized - for example to the thallus, the apothecia, the epinecrotic layer, or the cephalodia of their hosts.The host specificity of lecideicolous ascomycetes is extremely varied, ranging from species confined to a single host species or genus, to others which occur on a wide range of lecideoid and non-lecideoid lichens of diverse affinities. Some host-specific ascomycetes on members of the Lecideaceae s. str. and Porpidiaceae provide evidence for a co-evolutional origin of the lecideoid lichen species and their fungal inhabitants. The known lecideicolous fungi grow on lichens of extra-tropical regions. They are found in both hemispheres. A key for the identification of lecideicolous ascomycetes (with an English version) and a list of the lecideoid host lichens and their fungal inhabitants are provided. The lecideicolous ascomycetes treated in this paper belong to the following genera which are accepted as natural units: Arthonia, Carbonea, Cecidonia, Cercidospora, Clypeococcum, Dactylospora, Endococcus, Epilichen, Hymenobiella, Karschia, Lasiosphaeriopsis, Aierismatium, Muellerella, Opegrapha, Phaeopyxis, Pyrenidium, Rhymbocarpus, Sage- diopsis, Sphaerellothecium, Stigmidium, Zwackhiomyces. Sphaeria so- ciella could not be included in one of these genera. The genera are described and commented upon. Their lecideicolous species and varieties are taxonomically revised and represented in detail, with drawings of spore and ascus outlines provided for many descriptions of the accepted taxa. Thirty taxa are excluded: some because they are lichenized, some because they are not or only doubtfully lecideicolous. Fifteen names remain of uncertain application. Ten species are new to science: Arthonia obscurior Triebel, Cercidospora cephalodiorum Triebel & Grube, Dactylospora amygdalariae Triebel, D. australis Triebel & Hertel, D. polyspora Triebel, D. purpurascens Triebel, Epilichen stellatus Triebel, Opegrapha maligna Triebel, Sphaerellothecium abditum Triebel, S. contextum Triebel. The following new combinations are proposed: Dactylospora deminuta (Th. Fr.) Triebel, Hymenobiella aporea (Nyl.) Triebel, Lasiosphaeriopsis supersparsa (Zopf) Triebel, Merismatium decolorans (Arnold) Triebel, M. discrepans (Lahm) Triebel, Muellerella pygmaea var. athallina (Müll. Arg.) Triebel, M. pygmaea var. ventosicola (Mudd) Triebel, Opegrapha lamyi (Nyl.) Triebel, Rhymbocarpus elachistophorus (Nyl.) Triebel, R. gregarius (Sherw., D. Hawksw. & Coppins) Triebel, R. nitschkei (Koerber) Triebel, Sagediopsis aquatica (Stein) Triebel, S. barbara (Th. Fr.) R. Sant. & Triebel, Stigmidium conspurcans (Th. Fr.) Triebel & R. Sant., St. tabacinae (Arnold) Triebel. Hymenobiella Triebel (nomen novum) is introduced for Hymenobia Nyl. (nom. illeg.)