An implantable device system comprising: a first medical device (14; 40) which is a leadless cardiac pacemaker implantable entirely within the heart, comprising pacing means for delivering a pacing therapy to the heart of a patient; and a second medical device which is an extracardiac implantable device comprising sensing means to sense activity of the heart of the patient; wherein the first medical device (14; 40) also comprises first communications means for at least receiving communications from at least the second medical device, and the second medical device comprises second communications means for at least issuing communications to at least the first medical device (14; 40); wherein the second medical device comprises atrial event means for using the sensing means to detect an atrial event in the heart of the patient and, in response thereto, to issue a communication to the first medical device (14; 40); wherein the first medical device (14; 40) comprises therapy means for using the pacing means to deliver therapy to the heart of the patient in response to the first communication means receiving the communication issued by the second medical device; wherein the second medical device comprises initialization means for initializing the atrial event means, including: detection means for detecting at least first and second cardiac cycles of the patient; and window means for establishing a window for detection of the atrial event; wherein the atrial event means is configured to observe cardiac signals of the patient during the window for detection of the atrial event in order to detect the atrial event. a first medical device (14; 40) which is a leadless cardiac pacemaker implantable entirely within the heart, comprising pacing means for delivering a pacing therapy to the heart of a patient; and a second medical device which is an extracardiac implantable device comprising sensing means to sense activity of the heart of the patient; wherein the first medical device (14; 40) also comprises first communications means for at least receiving communications from at least the second medical device, and the second medical device comprises second communications means for at least issuing communications to at least the first medical device (14; 40); wherein the second medical device comprises atrial event means for using the sensing means to detect an atrial event in the heart of the patient and, in response thereto, to issue a communication to the first medical device (14; 40); wherein the first medical device (14; 40) comprises therapy means for using the pacing means to deliver therapy to the heart of the patient in response to the first communication means receiving the communication issued by the second medical device; wherein the second medical device comprises initialization means for initializing the atrial event means, including: detection means for detecting at least first and second cardiac cycles of the patient; and window means for establishing a window for detection of the atrial event; wherein the atrial event means is configured to observe cardiac signals of the patient during the window for detection of the atrial event in order to detect the atrial event. detection means for detecting at least first and second cardiac cycles of the patient; and window means for establishing a window for detection of the atrial event; wherein the atrial event means is configured to observe cardiac signals of the patient during the window for detection of the atrial event in order to detect the atrial event.