Holocarpic oomycetes are diverse and ubiquitous in aquatic environments. These organisms are mostly obligate biotrophic parasites, and almost all are members of the deep-branching early-diverging lineages of the Oomycota. Despite belonging to the protist kingdom Straminipila, these organisms have convergently acquired several fungal traits, such as osmotrophic nutrition. However, little is known about their biology and ecology despite their widespread nature and their potential importance as regulators of plankton blooms and pathogens of other oomycetes and plants. Most occurrences of holocarpic oomycetes have been reported decades ago, and only recently, some have been rediscovered and investigated for their molecular phylogeny. Here, we highlight the taxonomy, phylogeny, and diversity of the simple holocarpic oomycetes encountered in our on-going studies. These encompass species that are obligate endoparasitoids in diatoms (Coscinodiscus, Pseudo-nitzschia, Rhizosolenia, Pleurosigma, Gyrosigma, Licmophora, Synedra, Acnanthes, Nitzschia, Pinnularia, Ulnaria, Minidiscus, Craticula, Fragillaria, Navicula, Striatella, Melosira, Cymbella, Diploneis, Pleurosira), filamentous aquatic oomycetes (Saprolegnia, Achlya, Pythium, Globisporangium, Aphanomyces), algae (Ceramium, Capsosiphon, Polysiphonia, Chondrus, Palmaria, Spirogyra, Ancylistes, Oedogonium, Zygnema), freshwater invertebrates (Ostrocods, Rotifera), and plants (Pinus). These were isolated from various aquatic (marine, brackish water, freshwater) and terrestrial habitats in Germany, Norway, Iceland, Denmark, Italy, the Netherlands, Finland, Iran, and Mexico. A total of 49 newly described species and combinations, 3 new families, 3 new genera, and 4 new orders were introduced or reinstated in our recent and ongoing research. In addition, 13 rediscovered species and some recent insights into their diversity will be reported in this presentation.