work is licensed under a Creative Commons IGO 3.0 AttributionNonCommercial-NoDerivatives (CC-IGO BY-NC-ND 3.0 IGO) license (http://creativecommons.org/licenses/by-nc-nd/3.0/igo/ legalcode) and may be reproduced with attribution to the IDB and for any non-commercial purpose, as provided below. No derivative work is allowed.Any dispute related to the use of the works of the IDB that cannot be settled amicably shall be submitted to arbitration pursuant to the UNCITRAL rules. The use of the IDB's name for any purpose other than for attribution, and the use of IDB's logo shall be subject to a separate written license agreement between the IDB and the user and is not authorized as part of this CC-IGO license.Following a peer review process, and with previous written consent by the Inter-American Development Bank (IDB), a revised version of this work may also be reproduced in any academic journal, including those indexed by the American Economic Association's EconLit, provided that the IDB is credited and that the author(s) receive no income from the publication. Therefore, the restriction to receive income from such publication shall only extend to the publication's author(s). With regard to such restriction, in case of any inconsistency between the Creative Commons IGO 3.0 Attribution-NonCommercial-NoDerivatives license and these statements, the latter shall prevail.Note that link provided above includes additional terms and conditions of the license.The opinions expressed in this publication are those of the authors and do not necessarily reflect the views of the Inter-American Development Bank, its Board of Directors, or the countries they represent.http://www.iadb.org 2016 1 Abstract * This paper analyzes the impact of longer school days on student achievement in Colombia. To identify the impact of longer schools days, this study exploits plausibly exogenous within school variation in the length of the school day. Using test score data from 5 th and 9 th graders in 2002, 2005, and 2009, along with school administrative data, this research uses school fixed effects models to estimate variation in average test scores across cohorts for schools that switched from a half school day to a full school day or vice versa. I find that cohorts exposed to full school days have test scores that are about one tenth of a standard deviation higher than cohorts that attended half school days. The impact of attending full school days is larger for math than for language, and it is larger for 9 th grade than for 5 th grade. Effects are largest among the poorest schools and those in rural areas. The results suggest that lengthening the school day may be an effective policy for increasing student achievement, particularly for the lowest-income students in Colombia and other developing countries.JEL Classification: I21, I28, H43