This book was developed for the AAPM 2011 Summer School in Burnaby, British Columbia. The focus is on understanding the potential sources of dosimetric uncertainties in radiotherapy; evaluating the impact of these uncertainties on the accuracy and conformity of dose delivered to patients; understanding strategies for mitigating these uncertainties; and understanding the impact of these uncertainties on QA/QM. Summer School Program objectives are to provide in-depth understanding of sources of uncertainties in external beam radiotherapy planning and delivery; practical guidance in assessing the overall uncertainty of delivered dose to patients treated with different technologies; practical guidance on mitigating sources of uncertainties and strategies for dealing with residual uncertainties; and to impress upon the fact that ""What You See Is Not What You Get (WYSINWYG)"" and how to deal with it on patient-by-patient basis.