Terahertz frequencies match the vibrations between large functional groups in molecular networks from macromolecules, nano-droplets to proteins. If we are able to measure these oscillations we can decipher the structure and the long-range interactions in large molecular systems. This yields a precise fingerprint of the molecule that is highly useful for sensitive trace analysis. However, despite of a lot of research in the field, high precision spectroscopy in the former terahertz gap for isolated large molecular networks has not been developed yet.In this project I will develop the necessary tools to measure terahertz transition frequencies in large, mass-selected molecular systems with high resolution. For this purpose a cryogenic radiofrequency ion trap will be coupled to a terahertz resonator cavity. This will allow excitation of a dilute sample of molecular ions in well-defined internal quantum states with single-frequency terahertz radiation. My vision is to achieve high spectral resolution and single-ion sensitivity for almost arbitrarily large molecular systems in the terahertz regime which will initiate a new field for molecular spectroscopy.To explore the potential of the newly-developed methods, I propose to study molecular networks of fundamental importance in chemistry, biology and astronomy. Vibration-tunneling dynamics will be studied in water cluster ions. Torsional motion of biological chromophores and its role in the quenching of the fluorescent state will be investigated. And the spectral signatures of molecules that are promising candidates for detection in the interstellar medium will be determined.