1) Provide in-depth understanding of sources of uncertainties in external beam radiotherapy planning and delivery. 2) Provide practical guidance in assessing the overall uncertainty of delivered dose to patient treated with different technologies. 3) Provide practical guidance on mitigating sources of uncertainties and strategies for dealing with residual uncertainties. 4) 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.