This is the live session included as part of the LonTI lecture on Conformal blocks in two dimensions. Please register at https://lonti.weebly.com/registration.html to receive joining instructions for this live session which will be held via Zoom. D-branes are a remarkable set of soliton-like objects in string theory which have played a critical role in many areas of string theory. Despite being non-perturbative, they can be understood exactly using world-sheet methods through the boundary state formalism. In this lecture, I introduce the notion of a boundary state and show how to use it to extract physical information about D-branes and the open strings that live on them.