The use of vegetation classification to define ecosystems is not new. However, the ability to define them with increasing accuracy and to make use of quantitative measures of vegetation to define states of change and degrees of divergence from earlier related ecosystems has improved greatly with the establishment of the National Vegetation Classification (NVC), a sample-driven, quantitative, peer-reviewed, inductive hierarchical system. This session, organized by the ESA Panel on Vegetation, considers the quantification and monitoring of novel ecosystems using the NVC and explores different vegetation-defined types of novel ecosystems, their characteristics, origins, and affects upon different regions of North America. Main types of novel terrestrial ecosystems include those in which introduced species have, with or without human interaction, shifted natural processes to alter prior species composition; those in which existing native species have increased due to indirect human influence, creating different conditions for the vegetation palette; those which have shown a decline of native key species due to pathogens; and those in which high biomass of introduced species have been added without any major process change. The talks begin with an overview by Keeler-Wolf, which sets the stage for how the National Vegetation Classification describes and tracks novel ecosystems-- using the hierarchy at different levels, structural definitions (layer dominance, cover, etc.), nomenclature (ruderal or semi-natural, for example), and methodology (data collection, re-sampling, and vegetation mapping). Following this introduction, speakers will discuss specific cases in which different kinds of novel ecosystems have originated in North America and have changed from former vegetation patterns. They discuss the type and rate of change, the events and the processes precipitating and perpetuating the change, and its effects on key occupants and patterns of biodiversity of these systems. They also discuss both natural limits of, and human efforts to alter novel vegetation expansion. A concluding talk (Long et al.) summarizes the detection and tracking of different types of novel vegetation through analysis of thousands of sample plots and vegetation mapping at the national level.