This is part one of a two-module series on estimation of observed precipitation. Through use of rich illustrations, animations, and interactions, this module provides an overview of the science of precipitation estimation using various measuring platforms. First, we define quantitative precipitation estimation (QPE) and examine technologies for remote sensing of QPE, including radar and satellite and the strengths and limitations of each. That is followed by an examination of the use of rain gauges for precipitation estimation and important issues to consider with rain gauge measurement. Finally we provide an introduction to the strengths and limitations of using precipitation climatology for QPE including PRISM.