We have reintroduced 140 captive-born golden lion tamarins (Leontopithecus r. rosalia) into remnants of Atlantic Coastal Forest in the state of Rio de Janeiro, Brazil since 1984. About 60% are lost in the first post-release year. Golden lion tamarins are predatory omnivores and extractive foragers. They live in relatively stable social groups using large, three-dimensional territories. These traits correlate with relatively large brains and extended parental care in mammals, suggesting that many survival-critical behaviors must be learned. Most tamarin reintroduction losses can be traced ultimately to deficiencies in identifying, locating, and processing appropriate food; interacting appropriately and effectively with conspecifics; selecting appropriate microhabitat (including nightly shelter); and locomoting and spatial orientation. We view these deficiencies as mainly cognitive, involving object manipulation, extractive foraging, discrimination learning, and cognitive mapping. There may be sensitive periods for learning some survival-critical skills, and observation learning may be involved. Neither formal pre-release training, nor allowing reintroduction candidates to live at liberty in wooded portions of zoos, has significantly affected survival. Intense post-release support allows some to survive and reproduce. There have been over 300 births in the wild to reintroduced tamarins and their offspring. These born-wild tamarins appear to be behaviorally competent and show high survivorship, suggesting detailed study of their behavioral ontogeny. Reintroduction can serve conservation, and provide an opportunity to study behavioral adaptation to drastically changed environments.