Upper Proterozoic rocks of the Avalon zone constitute much of the eastern basement of the New England Appalachians and have played a major role in its evolution. Avalonian rocks in southeastern New England are a composite of at least two distinct assemblages definable as terranes, the Esmond–Dedham and the Hope Valley. These terranes appear to have had differing interactions with rocks derived from the North American continent, with major tectonic modifications and/or transport occurring in late Paleozoic time. The Esmond–Dedham terrane lacks any clear previous relationships to other rocks in the region and probably was newly accreted during late Paleozoic time. The Hope Valley terrane is composed predominantly of basement gneisses and is exposed beneath extensive metasedimentary sequences of varying ages. Together with possibly related rocks elsewhere in New England (the Willimantic, Pelham, and Massabesic gneisses), the Hope Valley gneisses show evidence of late Paleozoic deformation and metamorphism. Although these rocks have been thought to share much of their history with overlying metasedimentary sequences (the Kearsarge–Central Maine synclinorium, Merrimack trough, and Putnam–Nashoba zone), several lines of evidence suggest that their contacts are tectonic and of late Paleozoic age. Each of these major rock assemblages potentially is a terrane, but with poor definition of the time when their present positions were attained. A late Paleozoic age for final assembly is possible, although all the rocks were not necessarily “exotic” relative to North America throughout the Paleozoic Era.