The northern Myanmar hill jungle and neighboring highlands found along the southeastern slopes of the Himalayas (Hkakabo Razi region, northern Kachin State, northernmost Myanmar) is almost untouched by humans, and can be considered as primary forest in the literal sense. Until the efforts reported on in this monograph, no ornithologist had the opportunity to visit this remote area since the late 1940s. Beginning in 1997, we made several visits to this extraordinary, species-rich area, and compiled an inventory of the regional avifauna while also studying the ecology, behavior, systematics, taxonomy, biogeography, and origin of the region’s birds. We report on these aspects for the 441 species found in the area and add data on species new to the area and those for which there are only a few representatives in the world’s specimen collections. In addition, we provide information on taxa that are likely endemic ( Jabouilleia naungmungensis ), suggest splits or revisions of subspecies and species, and describe two new subspecies from the area, Alcippe cinereiceps hkakaboraziensis ssp. nov. and Malacocincla abbotti kachinensis ssp. nov., based on plumage and morphometric differentiation. Last but not least, we analyze the species affinities (i.e. the biogeographic origin of the species occurring in Hkakabo Razi and surrounding areas). We found a lack of data in general for bird species in Southeast Asia, India and Tibet/China, but especially in the northern hill mountains of Kachin State around Hkakabo Razi National Park. Efforts by conservation organizations and ourselves has improved the situation, but much more work, particularly on ecology and global change effects, is warranted. We have added considerable information on the distribution, systematics, ecology, and biogeography of a number of species. Although much work remains to be done, our research provides significant new insights into the biology and biogeography of the region’s birds, and in particular, on delineation of major zoogeographic regions. We conclude that Ernst Mayr was correct in essence when he stated that the eastern border between the Oriental and Southeast Asian regions is roughly equivalent to the political boundary between Myanmar (Burma) and China. However, our work provides new understanding of the reasons, and challenges some of the fundamental assumptions on which the ‘zoogeographic region’ concept is based. In particular, such delineations often fail to take into account the geological and ecological history of a region. By evaluating in detail the subspecific relationships of the Hkakabo Razi rainforest avifauna, we were able to determine that the avifauna most similar is found in the temperate rainforests of northeastern India. We conclude that the likely reason for this similarity is that these two avifaunas shared the same refugium during the maximum of the last glacial period (18,000 years ago), and that other south and southeast Asian rainforest species shared different refugia during this time period, and probably during previous glacial events as well.