This document discusses the use of numbers and quantitative analysis to establish payment for ecosystem services (PES) schemes in the Andes region. It provides three examples of PES schemes in Colombia and Peru where science-based analysis was used to determine the monetary value of ecosystem services like water regulation. The schemes aimed to compensate upstream land users for adopting practices that maintain ecosystem services and benefit downstream users. The examples show how numbers can provide an objective basis for dialogue between different stakeholders. The document advocates for using integrated analyses of natural, economic and social factors to both maximize environmental benefits and address rural poverty concerns when establishing PES schemes.