This is the first of two volumes providing a comprehensive overview of the theoretical foundations of modern high-resolution molecular spectroscopy. They highlight the significant progress achieved in recent decades, driven largely by advances in computing power and laser technologies. These books bring together complementary chapters that explain how theoretical developments, computational methods and modeling approaches allow scientists to extract precise molecular information from complex spectra.

Key topics include the treatment of non-rigid molecules, large-amplitude motions, effective Hamiltonians and variational techniques, all of which support accurate interpretation of rotational, vibrational and rovibrational spectra in diverse molecular systems. Both books also explore emerging and specialized techniques, such as microwave three wave mixing for chiral analysis, spectroscopic networks for validating data and improved treatments of non-adiabatic effects. Finally, they emphasize the importance of collisional effects in real spectroscopic environments, presenting advanced models for line shapes, spectral broadening, shifts and line mixing based on classical, semi-classical and quantum mechanical approaches.