In patients with chronic heart failure, detecting signs and symptoms of worsening congestion early enough to prevent hospitalization is an ongoing challenge. Intrathoracic impedance monitoring is a new, device-based method for detecting fluid accumulation in the lungs. Small clinical trials have shown that it may provide an early warning of impending decompensation and may be useful when used in conjunction with routine clinical assessment in guiding diuresis during hospitalization. Additional large-scale, prospective, randomized clinical trials are currently underway to further evaluate its clinical utility. This review discusses the basic concepts underlying intrathoracic impedance monitoring, the clinical evidence supporting its use, and some common pitfalls to avoid when interpreting intrathoracic impedance data.