The first module in the LETRS series explores the reasons why many students have reading difficulties and explains how children learn to read. Case studies illustrate the progression of reading development; the influence of biological, genetic, cognitive, environmental, and instructional factors in learning to read; and the components of effective reading instruction. The “four-part” processing system is explored in detail. This module lays the foundation for future modules by building the knowledge and research base for participants. It provides an opportunity for participants to dive deeply into the processes of reading, to then be able to assist students with best practices in learning to read.