Evaluation parameters for free flap monitoring devices are used inconsistently, leading to considerable confusion about the quality and applicability of these devices. A comparison of different systems and different clinical series is almost impossible. The ultimate questions that need to be answered are those regarding the efficiency and the effectiveness of the system. A new tool consisting of two simple parameters that sufficiently and comparably describe the quality of monitoring devices is developed. The flap failure reduction rate describes the percentage of saved flaps (effectiveness). The revision success rate describes the efficiency. Literature reevaluation shows inconsistent results, although all authors describe a positive experience. This shows the limited value of the classical parameters. Larger studies have flap failure reduction rates of 5 to 12% (5 to 12% of monitored flaps are saved). Revision success rates of 75 to 90% prove that the system is efficient enough in daily use. Reevaluation of the smaller reported series result in lower parameters, which shows that there is a significant learning curve for this device. The new parameters alleviate the confusion surrounding evaluation of monitoring systems by giving specific information about effectiveness and efficiency. The benefits of the implanted Doppler probe can now be clearly described. However, in several studies the benefit of the system is overrated.