INTRODUCTION The Cladocera (Crustacea: Branchiopoda) are a group of microscopic crustaceans that are dominant in continental water bodies of almost every type (Dumont & Negrea, 2002; Forró et al., 2008). They have become model objects for evolutionary, genetic, and ecological investigations that use different methods and approaches (Lampert, 2011; Smirnov, 2014). The family Eurycercidae Kurz, 1875 emend. Dumont & Silva-Briano, 1998, with the single genus Eurycercus Baird, 1843, is a remarkable group of the order Anomopoda Sars, 1865 (Crustacea: Branchiopoda), currently including 19 formal taxa at the species-group level, published and available according to the ICZN (2000). Among these, 13 are regarded as valid at this moment. Some of them are among the largest representatives of the cladocerans. They inhabit the littoral zone (preferring areas covered by macrophytes) in permanent water bodies in Eurasia and North America, more rarely in Africa and South America (Frey, 1971; Smirnov, 1971, 1974) and sometimes have an important role in freshwater ecosystems. Eurycercids consume decomposing organic matter, bacteria, and algae and, at the same time, are important food items for some species of fish. Especially in Arctic waters they make up a significant element in the fish diet (Björnsson, ‎2001; Bekker, 2012). The remains of Eurycercus (head shields, postabdomens etc.) are well preserved in lake bottom deposits (Frey, 1959; Szeroczyńska & Sarmaja-Korjonen, 2007; Smirnov, 2011; Korosi & Smol, 2012), and due to this the eurycercids are also utilized in palaeolimnological reconstructions of palaeoclimate, water level changes, etc. But despite their significant role in freshwater ecosystems, many aspects of eurycercid biology have been studied inadequately. Even their taxonomy remained confused until the last decade. The first eurycercid was briefly described as Lynceus lamellatus in the first major paper of the pioneer of cladoceran studies, O. F. Müller (1776). Later, he gave a more detailed redescription and realistic illustration of this taxon (O. F. Müller, 1785). Baird (1843) placed it to a separate subgenus of the genus Lynceus O. F. Müller, 1776, and increased the rank of Eurycercus Baird, 1843 to that of a separate genus (Baird, 1845). Lilljeborg (1887) found a new species, E. glacialis Lilljeborg, 1887, at Bering Island near Kamchatka Peninsula. Since that time eurycercids were recorded countless times in the literature of the late 19th century – early 20th century (Sars, 1861(1993); Lilljeborg, 1901; Herrick, 1884; Fordyce, 1901; Dybowski & Grochowski, 1898). Hellich (1877) was the first author who described dorsal head pores in some cladocerans which would soon be placed in the families Chydoridae and Eurycercidae (both families were not established at that time and regarded as members of the family Lynceidae Baird, 1845). During a long time, from the end of the 19th century until the 1970th, the majority of the authors considered (Behning, 1941; Manujlova, 1964) that the genus included only two species, E. lamellatus and E. glacialis. This compromises the value of faunistic, zoogeographical and ecological publications mentioning only these two eurycercids. Some new taxa proposed during that time (Dybowski & Grochowski, 1898; Birabén, 1939) were described inadequately and were not regarded as valid by subsequent authors. A new period of eurycercid study was opened by David Frey (1971, 1973a–b, 1975a–b, 1978) who analysed in detail their morphology, described several new species, and proposed a series of new diagnostic characters such as head shield shape, labral keel shape and length, etc. He also proposed an intergeneric subdivision of the genus. Studies were continued by Duigan (Duigan & Frey, 1987a–b; Duigan, 1991) and Hann (1980, 1982, 1985, 1990). After the 1990s, interest in eurycercid taxonomy was lost until the last decade, when a series of publications appeared (Aranguren et al., 2010), and the authors of this book contributed to resolving some taxonomic problems concerning eurycercids (Kotov, 2000; Bekker et al., 2010, 2012, 2016). Frey (1967) proposed a subdivision of the family Chydoridae Dybowski et Grochowski, 1894 into four subfamilies, i.e. Eurycercinae (note that subfamily Eurycercinae was already suggested by Kurz (1875) and must be named as Eurycercinae Kurz, 1875, see below). Then the rank of this taxon was raised up to a family (Dumont & Silva-Briano, 1998), an approach supported by further investigators (see Kotov 2013). In the present book we also regard the Eurycercidae as an independent family. This monograph is an attempt to summarize the available data on taxonomy and distribution of all currently known species of the genus. In contrast to a previous volume in this series on Ilyocryptus (Kotov & Štifter, 2006), we can confidently state that only few additional taxa of Eurycercus will be discovered in the future, i.e. in inadequately studied Arctic Canada, where few undescribed taxa of E. (Teretifrons) occur according to Hann (1990).