This document discusses dose-volume histograms (DVHs) which are used to analyze and compare radiation dose distributions in radiotherapy treatment planning. It describes how DVHs are generated by counting the number of voxels receiving different dose levels. DVHs can be displayed cumulatively or differentially and show the volume of structures receiving particular doses. The document outlines some limitations of DVHs including their insensitivity to small hot or cold spots and lack of spatial information. It emphasizes that DVHs should be used along with visual analysis of dose distributions and dose-volume statistics when evaluating treatment plans.