An implantable medical device operable for delivery of cardiac therapy to a patient, the device comprising: a therapy delivery module configured to deliver pacing therapy to either the left ventricle or the right ventricle of a patient's heart using at least one electrode; a sensing module configured to sense electrical activity of the patient's heart using at least one electrode; and a control module coupled to the therapy delivery module and to the sensing module and configured to: adjust an AV delay; control the delivery of pacing therapy to either the left ventricle or the right ventricle of a patient's heart based on the AV delay, wherein the pacing therapy is delivered over a plurality of heartbeats, sense electrical activity of the patient's heart using the sensing module during the delivery of the pacing therapy, measure a ventricular activation time for each of the plurality of heartbeats between the delivery of pacing stimulus of the pacing therapy and at least one selected fiducial point of the sensed electrical activity resulting from at least one of the delivered pacing stimulus of the pacing therapy and an intrinsic conduction of the patient's heart, and modify the AV delay for use in delivering pacing therapy based on the measured ventricular activation times if one or more of the measured ventricular activation times are greater than or less than a predetermined reference activation time by a selected threshold value; and characterised in that the control module is configured to modify the AV delay for use in delivering pacing therapy based on the measured ventricular activation times, the control module is further configured to set the AV delay to the last AV delay plus the last measured activation time minus the predetermined reference activation time. a therapy delivery module configured to deliver pacing therapy to either the left ventricle or the right ventricle of a patient's heart using at least one electrode; a sensing module configured to sense electrical activity of the patient's heart using at least one electrode; and a control module coupled to the therapy delivery module and to the sensing module and configured to: adjust an AV delay; control the delivery of pacing therapy to either the left ventricle or the right ventricle of a patient's heart based on the AV delay, wherein the pacing therapy is delivered over a plurality of heartbeats, sense electrical activity of the patient's heart using the sensing module during the delivery of the pacing therapy, measure a ventricular activation time for each of the plurality of heartbeats between the delivery of pacing stimulus of the pacing therapy and at least one selected fiducial point of the sensed electrical activity resulting from at least one of the delivered pacing stimulus of the pacing therapy and an intrinsic conduction of the patient's heart, and modify the AV delay for use in delivering pacing therapy based on the measured ventricular activation times if one or more of the measured ventricular activation times are greater than or less than a predetermined reference activation time by a selected threshold value; and characterised in that the control module is configured to modify the AV delay for use in delivering pacing therapy based on the measured ventricular activation times, the control module is further configured to set the AV delay to the last AV delay plus the last measured activation time minus the predetermined reference activation time. adjust an AV delay; control the delivery of pacing therapy to either the left ventricle or the right ventricle of a patient's heart based on the AV delay, wherein the pacing therapy is delivered over a plurality of heartbeats, sense electrical activity of the patient's heart using the sensing module during the delivery of the pacing therapy, measure a ventricular activation time for each of the plurality of heartbeats between the delivery of pacing stimulus of the pacing therapy and at least one selected fiducial point of the sensed electrical activity resulting from at least one of the delivered pacing stimulus of the pacing therapy and an intrinsic conduction of the patient's heart, and modify the AV delay for use in delivering pacing therapy based on the measured ventricular activation times if one or more of the measured ventricular activation times are greater than or less than a predetermined reference activation time by a selected threshold value; and characterised in that the control module is configured to modify the AV delay for use in delivering pacing therapy based on the measured ventricular activation times, the control module is further configured to set the AV delay to the last AV delay plus the last measured activation time minus the predetermined reference activation time.