This document provides an overview of energy efficient algorithms in wireless sensor networks (WSNs). It discusses how WSNs work, including how sensor nodes communicate wirelessly to monitor environments. To reduce energy consumption, WSNs often use clustering techniques where nodes are organized into clusters with a cluster head that aggregates data from nodes in its cluster. The document reviews several common energy efficient routing algorithms for WSNs that utilize clustering, including LEACH, TEEN, SEP, DEEC, and others. It also discusses challenges for WSNs like limited energy resources, security, scalability, and fault tolerance.