A medical device system for controlling delivery of cardiac resynchronization therapy, the system comprising: therapy delivery circuitry configured to deliver ventricular pacing to a heart of a patient; sensing circuitry configured to sense electrical activity of the heart via a plurality of electrodes (10, 66, 82, 135); and one or more processors configured to: control the therapy delivery circuitry to deliver the ventricular pacing according to a sequence of different values of at least one of A-V delay or V-V delay; during the delivery of ventricular pacing according to the sequence, control the sensing circuitry to acquire one or more electrograms, each of the one or more electrograms from a respective one of a plurality of vectors formed by the plurality of electrodes; for each of the different values of the at least one of A-V delay or V-V delay, determine at least one of a QRS amplitude or a QRS area based on the one or more electrograms; identify a target change in QRS amplitude or QRS area between a first value of the sequence of different values of the at least one of A-V delay or V-V delay and a second value of the sequence of different values of the at least one of A-V delay or V-V delay, wherein the second value is one of a next-shortest value or a next-longest value, relative to the first value, of the sequence of different values of the at least one of A-V delay or V-V delay; and in response to the identification of the target change, control the therapy delivery circuitry to deliver the ventricular pacing at a value of the at least one of A-V delay or V-V delay determined based on the identification of the target change to provide CRT. therapy delivery circuitry configured to deliver ventricular pacing to a heart of a patient; sensing circuitry configured to sense electrical activity of the heart via a plurality of electrodes (10, 66, 82, 135); and one or more processors configured to: control the therapy delivery circuitry to deliver the ventricular pacing according to a sequence of different values of at least one of A-V delay or V-V delay; during the delivery of ventricular pacing according to the sequence, control the sensing circuitry to acquire one or more electrograms, each of the one or more electrograms from a respective one of a plurality of vectors formed by the plurality of electrodes; for each of the different values of the at least one of A-V delay or V-V delay, determine at least one of a QRS amplitude or a QRS area based on the one or more electrograms; identify a target change in QRS amplitude or QRS area between a first value of the sequence of different values of the at least one of A-V delay or V-V delay and a second value of the sequence of different values of the at least one of A-V delay or V-V delay, wherein the second value is one of a next-shortest value or a next-longest value, relative to the first value, of the sequence of different values of the at least one of A-V delay or V-V delay; and in response to the identification of the target change, control the therapy delivery circuitry to deliver the ventricular pacing at a value of the at least one of A-V delay or V-V delay determined based on the identification of the target change to provide CRT. control the therapy delivery circuitry to deliver the ventricular pacing according to a sequence of different values of at least one of A-V delay or V-V delay; during the delivery of ventricular pacing according to the sequence, control the sensing circuitry to acquire one or more electrograms, each of the one or more electrograms from a respective one of a plurality of vectors formed by the plurality of electrodes; for each of the different values of the at least one of A-V delay or V-V delay, determine at least one of a QRS amplitude or a QRS area based on the one or more electrograms; identify a target change in QRS amplitude or QRS area between a first value of the sequence of different values of the at least one of A-V delay or V-V delay and a second value of the sequence of different values of the at least one of A-V delay or V-V delay, wherein the second value is one of a next-shortest value or a next-longest value, relative to the first value, of the sequence of different values of the at least one of A-V delay or V-V delay; and in response to the identification of the target change, control the therapy delivery circuitry to deliver the ventricular pacing at a value of the at least one of A-V delay or V-V delay determined based on the identification of the target change to provide CRT.