Our understanding of migratory birds’ year-round ecology and evolution remains patchy despite recent fundamental advances. Periodic reviews focus future research and inform conservation andmanagement; here, we take advantage of our combined experiences working on Western Hemisphere avian migration systems to highlight recent lessons and critical gaps in knowledge. Among topics discussed are: (1) The pipeline from pure to applied researchers leaves room for improvement. (2) Population limitation and regulation includes both seasonal and between-season interactions. (3) The study of movements of small-bodied species remains a major research frontier. (4) We must increase our understanding of population connectivity. (5)With few exceptions, population regulation has barely been investigated. (6)We have increasingly integrated landscape configuration of habitats, large-scale habitat disturbances, andhabitat quality impacts intomodels of seasonal andoverall demographic success. (7)The post-breeding season (late summer for latitudinal migrants) is increasingly appreciated for its impacts on demography. (8) We recognize the diverse ways that avian brood parasites, nest predators, and food availability affect demography. (9) Source–sink and meta-population models help us understandmigratory avian distributions among fragmented habitats. (10) Advances in modeling have improved estimates of annual survival and fecundity, but for few species. (11) Populations can be limited by ecological conditions in winter, but habitat needs are poorly known for most species at this time. (12) Migration tends to occupy broad spatial fronts that may change seasonally or whenmigrants crossmajor barriers. (13) Manuscript received 6 March 2009; accepted 20 April 2009. Corresponding Editor: J. M. Marzluff. 18 E-mail: firstname.lastname@example.org 19 Present address: Natural Resources Branch, EV2 Environmental Planning, NAVFAC Pacific, 258 Makalapa Drive, Suite 1000, Pearl Harbor, Hawaii 96860-3134 USA. 20 Present address: Wildlife Health Center, School of Veterinary Medicine, University of California, Davis, California 95616 USA.