Abstract: Why are take up rates incomplete or low when the relevant opportunities are unambiguously advantageous to people who are eligible for them? How can public officials promote higher take up of opportunities? All over the world, these are challenges of the first order. There are three primary barriers to take up: learning costs, compliance costs, and psychological costs. These costs lower the net expected benefit of opportunities, and reduce participation in otherwise advantageous programs. Fully rational agents would consider these costs in their take up decisions, and in light of behavioral biases, such costs loom especially large and may seem prohibitive. Experimental and other evidence suggest methods for reducing the barriers to take up and the effects of behavioral biases. Use of such methods has the potential to significantly increase access to a wide range of opportunities that would increase individual well-being and social welfare.