The primary objective of the first-generation implantable cardiac pacemakers was to provide critical heart rate support, but these devices did not have any diagnostic capabilities. In the intervening decades, the number, type, and complexity of implantable devices has greatly expanded. Today, implantable devices not only provide heart rate support but they also provide protection from sudden cardiac death with implantable cardioverter defibrillators (ICDs) and reduce symptoms and increase survival with cardiac resynchronization therapy (CRT). Furthermore, information on physiologic variables has been collected in patients with implanted devices for the purpose of providing sophisticated closed-loop optimization of their pacing and defibrillation algorithms. Thoracic fluid status monitoring via intrathoracic impedance is the newest device-based diagnostic capability. For those patients with heart failure who are already targeted to receive an ICD or CRT with defibrillator implant, the ability to monitor fluid status can provide additional insight into the difficult problem of evaluating and managing these patients. This article reviews the basics of measuring intrathoracic impedance via OptiVol fluid status monitoring (Medtronic, Inc., Minneapolis, MN), as well as clinical results regarding the utility of evaluating OptiVol intrathoracic impedance data trends.