Recent progresses and applications of resonance-enhanced multiphoton ionization (REMPI) spectroscopy are reviewed. In our laboratory, REMPI spectroscopy has been applied to study the Rydberg states of propyne, allene, vinyl chloride, and allyl radical. Based on two-photon theory, polarization-ratio measurements using circularly and linearly polarized lasers were applied to reveal the symmetries of molecular excited states. By fitting the term values of the observed Rydberg series to Rydberg formula, the adiabatic ionization energies of propyne, vinyl chloride, allyl-h₅ radical, and allyl-d₅ radical are accurately determined to be 83625 ± 2, 80720 ± 6, 65594 ± 9, and 65527 ± 16 cm^-1, respectively. Ab initio calculation of Franck-Condon factors is demonstrated to be a powerful tool for spectral analysis of the congested vibronic transitions to Rydberg states. The advantages of using the REMPI technique for scientific research and some practical applications are discussed.