This thesis reports the effect of applying static and ramped electric fields to quasi-bound Rydberg states of NO, with principal quantum number n = 25 - 32. The Rydberg states are excited by double resonance via the v'= 0,/V' = 0, and v' = 0,N' = 2 rovibrational states of the intermediate A2!' state of NO. Spectroscopic data is obtained by application of a static electric field ranging from 0 to 129 V cm"1, and ramped electric fields. In the presence of DC electric fields, the experimental Stark spectra presented in this thesis reveal a number of new interesting features, simulated using a matrix-diagonalisation approach. In this calculation, the adjustable parameters are the dipole transition moments from the various angular momentum components of the / -state, A, (/), for which just one set is used to obtain the qualitative agreement with the experimental spectra via two different rotational states of the / -state. The first detailed investigations of the selective field ionisation (SFI) of Rydberg states in a molecule are presented. The competition between electron-nuclear coupling and electron-field coupling is investigated and it is shown that the slew rate of the applied electric field can be exploited to control the rotational quantum state composition of field-ionised molecules.