An implantable medical device comprising: a plurality of electrodes comprising: a right atrial electrode positionable within the right atrium to deliver cardiac therapy to or sense electrical activity of the right atrium of the patient's heart; and at least one left ventricular electrode positionable proximate the left ventricle of the patient's heart to deliver cardiac therapy to or sense electrical activity of the left ventricle of the patient's heart; a housing extending from a proximal end region to a distal end region, wherein the right atrial electrode is leadlessly coupled to the proximal end region; a leadlet extending from a proximal region to a distal region, wherein the proximal region is coupled to the distal end region of the housing and the at least one ventricular electrode is coupled to the distal region of the leadlet, wherein the leadlet is configured to extend through the coronary sinus ostium and into the coronary sinus or a coronary vein of the patient's heart to position the at least one left ventricular electrode proximate the left ventricle of the patient's heart; a therapy delivery circuit within the housing and operably coupled to the plurality of electrodes to deliver cardiac therapy to the patient's heart; a sensing circuit within the housing and operably coupled to the plurality of electrodes to sense electrical activity of the patient's heart; and a controller within the housing and comprising processing circuitry operably coupled to the therapy delivery circuit and the sensing circuit, the controller configured to: monitor electrical activity using the processing circuitry and one or more of the plurality of electrodes; and delivering pacing therapy using the processing circuitry and one or more of the plurality of electrodes. a plurality of electrodes comprising: a right atrial electrode positionable within the right atrium to deliver cardiac therapy to or sense electrical activity of the right atrium of the patient's heart; and at least one left ventricular electrode positionable proximate the left ventricle of the patient's heart to deliver cardiac therapy to or sense electrical activity of the left ventricle of the patient's heart; a right atrial electrode positionable within the right atrium to deliver cardiac therapy to or sense electrical activity of the right atrium of the patient's heart; and at least one left ventricular electrode positionable proximate the left ventricle of the patient's heart to deliver cardiac therapy to or sense electrical activity of the left ventricle of the patient's heart; a housing extending from a proximal end region to a distal end region, wherein the right atrial electrode is leadlessly coupled to the proximal end region; a leadlet extending from a proximal region to a distal region, wherein the proximal region is coupled to the distal end region of the housing and the at least one ventricular electrode is coupled to the distal region of the leadlet, wherein the leadlet is configured to extend through the coronary sinus ostium and into the coronary sinus or a coronary vein of the patient's heart to position the at least one left ventricular electrode proximate the left ventricle of the patient's heart; a therapy delivery circuit within the housing and operably coupled to the plurality of electrodes to deliver cardiac therapy to the patient's heart; a sensing circuit within the housing and operably coupled to the plurality of electrodes to sense electrical activity of the patient's heart; and a controller within the housing and comprising processing circuitry operably coupled to the therapy delivery circuit and the sensing circuit, the controller configured to: monitor electrical activity using the processing circuitry and one or more of the plurality of electrodes; and delivering pacing therapy using the processing circuitry and one or more of the plurality of electrodes. monitor electrical activity using the processing circuitry and one or more of the plurality of electrodes; and delivering pacing therapy using the processing circuitry and one or more of the plurality of electrodes.