Over the past forty years, investigations of radio recombinationlines have become an effective way of studying the interstellarmedium. The electron temperatures, densities, emission measures,kinematics parameters, elemental abundances and other physicalcharacteristics of emission processes in planetary nebulae,distributed ionized gas, stellar winds, the partially-ionizedinterstellar medium and external galaxies are determined with thelines of hydrogen, helium, carbon, etc. This astrophysicalphenomenon is unique because of the large (>1000) number ofpossible quantum transitions and because of the wide observablefrequency range (from ~150 GHz to ~10 MHz). In thisshort review the main results from recombination radio lineinvestigations are discussed. Special attention is given to theareas which have been given least attention in the literature suchas the radio recombination lines of very excited carbon atoms atthe extremely low-frequency (v < 30 MHz) end of the decameterwavelength range. This is important to stress in the 70-th jubileeof radio astronomy because cosmic radio emission was discovered byKarl Jansky at decameter wavelengths. Investigations of theextremely low-frequency radio recombination lines are also veryinteresting for the prospective LOFAR system.