This document provides an overview of Adhearsion, an open source telephony framework for building multimodal applications. It discusses how Adhearsion uses the Asterisk telephony engine to enable voice applications and provides APIs for REST, JSON, and other protocols. Examples are given of applications built with Adhearsion including a voice/Twitter voting app. The document also introduces Tropo, a cloud telephony API that can be used with Adhearsion's programming interfaces.