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Blood parasites of the genus <i>Leucocytozoon</i> (Leucocytozoidae) only inhabit birds and represent a readily distinct evolutionary branch of the haemosporidians (Haemosporida, Apicomplexa). Some species cause pathology and even severe leucocytozoonosis in avian hosts, including poultry. The diversity of <i>Leucocytozoon</i> pathogens is remarkable, with over 1400 genetic lineages detected, most of which, however, have not been identified to the species level. At most, approximately 45 morphologically distinct species of <i>Leucocytozoon</i> have been described, but only a few have associated molecular data. This is unfortunate because basic information about named and morphologically recognized <i>Leucocytozoon</i> species is essential for a better understanding of phylogenetically closely related leucocytozoids that are known only by DNA sequence. Despite much research on haemosporidian parasites during the past 30 years, there has not been much progress in taxonomy, vectors, patterns of transmission, pathogenicity, and other aspects of the biology of these cosmopolitan bird pathogens. This study reviewed the available basic information on avian <i>Leucocytozoon</i> species, with particular attention to some obstacles that prevent progress to better understanding the biology of leucocytozoids. Major gaps in current <i>Leucocytozoon</i> species research are discussed, and possible approaches are suggested to resolve some issues that have limited practical parasitological studies of these pathogens.