An essential element in the eddy current testing methodology is the use of multiple testing frequencies. A true volumetric inspection requires complete penetration of the tube wall from various perspectives. Since prime frequency is set to approximately thirty seven percent standard depth of penetration of the tube wall, additional frequencies are required to gain an accurate and detailed perspective of the defect. In fact, the need for additional frequencies is absolutely necessary to characterize particular defects such as under-deposit corrosion and microbiologically influenced corrosion. In such cases a minimum of three frequencies are required to perform optimum testing. The frequencies will vary from low, midrange to high depending on the material under test. A fourth frequency can also be used to identify tube supports and tubesheets. This paper demonstrates the effects and advantages of utilizing multiple frequencies in the testing process. The result of which is to provide the analyst with sufficient information to call the defects with greater accuracy and reliability. Utilizing a multi-frequency test instrument and standard probes commonly used for testing condenser tubes, tube samples with various defects will be examined utilizing one, two, three and four frequencies. Defect acquisition rates will also be varied and the results reported. The information is useful for determining the number of frequencies required for an effective eddy current test.