High-resolution spectroscopic techniques, in particular microwave and infrared diode laser spectroscopy, have been applied to detect, identify and characterize transient molecules such as free radicals and molecular ions in the gas phase through the observation of their rotational, rovibrational and/or rovibronic spectra. Some recent results are discussed in detail. Kinetic spectroscopy has been developed in the infrared and microwave regions to clarify chemical and physical processes involving transient molecules; photochemical and photo-induced reactions are investigated by recording the spectra of intermediate species with this technique in real time. Two applications of spectroscopy of transient molecules are mentioned; one is detection and identification of interstellar molecules and the other is development of a means for diagnosing discharge plasmas such as those employed for making thin films of amorphous silicon.