A novel ecosystem is one that substantively differs in composition or function from all ecosystems present in a reference domain. Novel climates can be similarly defined as climatic conditions unlike any in a reference domain. Usually the present or recent past provides that reference domain. Novel climates can produce novel ecosystems, as species respond individualistically to the opening up of new portions of environmental space. We live in a world of rising novelty. Many contemporary ecosystems already differ substantively from all historic counterparts, due to species introductions, land use, species extinctions, altered nutrient cycling, etc. Future ecosystems are expected to differ from those found today, as species differentially lag rapid climate change, and as novel climates emerge and species occupy previously unavailable portions of their fundamental niches. Novel ecosystems and climates pose both opportunities and challenges for ecologists. Novel ecosystems provide new systems for testing ecological theory, and may provide needed services. However, the expected emergence of future novel climates and ecosystems seriously challenges ecological forecasting, and increases the likelihood of ecological surprises. Additionally, conservation biologists debate whether novel ecosystems have less conservation value than ecosystems closer to historic counterparts. Our expectation that current climate trends will produce novel ecosystems is strongly informed by the paleohistorical record. Many communities with no modern analog are documented in the Quaternary fossil record, and many of these no-analog communities have been linked to environmental also lacking modern analogs. Paleohistorical research helps answer several critical questions related to novel ecosystems and climates: 1) How novel are future projected conditions? Are future conditions novel relative to only the 20th century, or do they differ from any during past interglacial-glacial cycles? Are future conditions evolutionarily novel, i.e. unlike any experienced by a species during its evolutionary lifetime? 2) Do ecological processes and patterns observed today hold true for past novel communities and climates? 3) Why have communities with no modern analog formed in the past? Explanations include the emergence of no-analog climates, rapid environmental change causing disequilibrial community dynamics, and trophic cascades caused by Pleistocene megafaunal extinctions. 4) How well can ecological forecasting models hindcast the distribution, composition, and structure of past novel ecosystems? Given tests against benchmarks datasets from the paleorecord, how much confidence should we place in predictions of future ecosystems? This symposium weaves together a mixture of theoretical and empirical perspectives and case studies drawn from both the contemporary and paleohistorical literature.