In order to image and to record local spectra of the surfaces of insulating films and solids, we have developed a tunable alternating current scanning tunneling microscope (ACSTM). This microscope has a bias voltage modulation frequency tunable over the range 0-20 GHz. We have recorded images and spectra using the amplitudes at the modulation frequency (f(0)) and its harmonics (nf(0)) on the surfaces of conductors, semiconductors, and insulators. We report the first atomic resolution images and nonlinear spectra using an ACSTM.