Closed-loop control of the glucose concentration in type 1 diabetes has been the subject of extensive research over the last 3 decades. Building on the recent progress in continuous glucose sensing techniques, several prototypes of a closed-loop system have been developed. To complement existing measures of glucose control, we designed a grading system specifically designed to provide clinical assessment of closed-loop systems including that of glucose controllers. The system introduces six grades, A-F, describing the level of control and the therapeutic intervention during outside-meal and postprandial conditions. Grades A and B represent excellent and good glucose control, respectively, without the need for a corrective therapeutic action. Grade C represents suboptimal control with a recommendation for a corrective action. Grade D represents poor control requiring a corrective action. Grades E and F represent very poor and life-threatening control, respectively, with a need for an immediate corrective action or requiring external assistance. The outcome of grading is the quantification of time spent in each grade. The grading system is exemplified using data obtained with a model predictive controller within an in silico simulation environment. We conclude that the grading system provides suitable means to assess efficacy and safety of glucose controllers complementing existing measures of glucose control.