Why do some individuals fail to collect the social or private benefits for which they are eligible?  We address this puzzle, and attempt to identify concrete strategies to increase take-up, with a unique field experiment in collaboration with the IRS.  Specifically, we test the role of program information (regarding benefits, costs, and rules), informational complexity, and stigma on response to a set of experimental IRS mailings that notify eligible individuals of over $26 million in owed benefits.  We distribute these mailings to 35,050 California taxpayers who filed their taxes but did not claim their TY 2009 EITC despite presumed eligibility.  Among eligible filers, we find residual increases in take-up due to the mere receipt of an experimental mailing (response of 0.14); simplification (+0.09 relative to 0.14); and the display of benefit information (+0.08 relative to the 0.23 simplified baseline).  We find no evidence to implicate perceptions of direct transaction costs, audit likelihood, or program stigma, as causes of incomplete take-up in this context.  The mailings also substantially increase the likelihood of subsequent year take-up.  A novel survey affirms pervasive low awareness and misconstrual of program incentives among eligible tax-filers, while psychometric evidence illuminates possible underlying mechanisms. We calculate that treatments, if applied to the entire population of filing non-claimants, could reduce overall incomplete take-up from 25% to 22%.