We review experiments carried out at Stony Brook during the past three years on (de-)excitation and ionization of highly-excited hydrogen atoms by microwave electric fields. These include single-frequency experiments in both the low- and high-scaled-frequency regime and two- frequency experiments in the low-scaled-frequency regime. We compare these experimental results with ”3-d“ atoms to the rapidly expanding body of calculations using both classical and quantal theory. We offer a brief comparison of laser- and microwave-ionization experiments with free atoms. © 1989 IOP Publishing Ltd.