This paper describes the Carotid Lumen Segmentation and Stenosis Grading Challenge, which was partof the workshop '3D Segmentation in the Clinic: A Grand Challenge'. This workshop was part of the12th conference on Medical Image Computing and Computer Assisted Interventions, held in Londonin September 2009. We introduce the purpose and background of the challenge and describe the data,reference standard and evaluation measures that were used in this challenge. We also demonstrate theevaluation framework by applying it to the data that were submitted by industrial and academic researchinstitutes.