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The use of multiple frequencies in a nonlinear microwave algorithm is considered. Using multiple frequencies allows for obtaining the improved resolution available at the higher frequencies while retaining the regularizing effects of the lower frequencies. However, a number of different challenges arise when using data from multiple frequencies for imaging of biological targets. In this paper, the performance of a multi-frequency algorithm, in which measurement data from several different frequencies are used at once, is compared with a stepped-frequency algorithm, in which images reconstructed at lower frequencies are used as starting guesses for reconstructions at higher frequencies. The performance is illustrated using simulated 2-D data and data obtained with the 3-D DTU microwave imaging system.