Quantum-logic techniques for state preparation, manipulation and non-destructive interrogation are increasingly being adopted for experiments on single molecular ions confined in traps. The ability to control molecular ions on the quantum level via a co-trapped atomic ion offers intriguing possibilities for new experiments in the realms of precision spectroscopy, quantum information processing, cold chemistry and quantum technologies with molecules. The present article gives an overview of the basic experimental methods, recent developments and prospects in this field.