The formation of stars and planetary systems takes place in “molecular clouds”. These dense, cold regions of the interstellar medium, composed primarily of molecular hydrogen, exhibit nevertheless a high degree of molecular complexity. About one third of the approximately 200 known interstellar and circumstellar molecules are the so-called “complex organics”, defined as molecules containing 6 or more atoms. How this molecular complexity develops, and how far it progresses before the molecules are incorporated as ices into planetesimals in protoplanetary disks, are key questions of today’s astrochemistry, which studies the molecular complexity through a combination of astronomical observations, theoretical computations, and laboratory experiments. Both gas-phase and grain-surface processes play an important role, and understanding the interactions between the gas phase and the ice surfaces is of key importance. Molecular hot cores and hot corinos, dense, compact regions heated by UV photons from newly formed stars, where mantle ices evaporate into the gas phase are of particular importance, as the molecules produced by grain-surface reactions can be studied by means of the high-resolution rotational spectroscopy. I review recent progress in our understanding of these regions made through observations using the Atacama Large Millimeter/submillimeter Array (ALMA).