Chapter 1: Introduction to WFC3 1.1 Overview 1.2 Key Features of WFC3 1.3 WFC3 Quick Reference Guide 1.4 Special Considerations for Cycle 21 1.4.1 Current Instrument Status 1.4.2 Ongoing Calibration 1.4.3 New Capabilities 1.5 Sources of Further Information 1.6 The WFC3 Instrument Team at STScI