This document provides an introduction to Geographic Information Systems (GIS). It begins with definitions of GIS, describing it as a computer system for capturing, storing, analyzing and displaying spatially-referenced data to support decision-making. The document then discusses the historical development of GIS from earlier concepts of digital cartography and database management. It also outlines the key components of a GIS, including hardware, software, data, users and methods of analysis. A variety of data types that can be used in GIS are described as well as what makes data considered "spatial".