Development planning was a form of interventionist social knowledge widely used in the mid-twentieth century. Planning was employed with different aims, and the adoption of concrete techniques and procedures was highly sensitive to each country’s institutional context. This article studies the life trajectory of Colombia’s Ten-year Plan, an internationally celebrated attempt to design economic development on a large scale in what actors characterized as a politically “democratic” and economically “liberal” setting. Based on the Colombian case, I argue that a central function of planning in developing countries was to build trust, on behalf of local stakeholders and international donors, in the state’s capacity to credibly use public resources and foreign aid to achieve its development aims. In turn, planning also allowed outsiders to invigilate the actions taken by states on the economy, and to make them accountable for their commitments. I examine the media of persuasion used in the build-up to, and the publicization and revision of the Ten-year Plan, to account for the shift from the macro scale of comprehensive plans to the smaller-scale development interventions observed in the 1960s. This case shows that the malleability of planning procedures was key for the enduring resilience of the planning system.