Studies of cardiovascular signals continuously sensed by implantable devices provide unique insight into detailed pathophysiology as patients progress from stable to congested states. These data suggest that volume expansion, autonomic adaptation, and pulmonary interstitial edema begin several weeks before patients develop symptoms or demonstrate changes in daily weight. Monitoring physiologic signals from implanted devices may provide earlier warning of impending decompensation, thereby allowing changes in medical therapy to prevent worsening heart failure.