This thesis comprises of a set of time and energy-resolved photoelectron spectroscopy studies of polyatomic molecules exposed to radiation ranging from the IR to soft X-ray region of the spectrum. Chapter 1 provides an introduction to molecular photochemistry, wavepacket dynamics and ionisation mechanism ranging from weak single photon ionisation to strong-field ionisation in intense laser fields. In Chapter 2 an experiment to probe excited state wavepacket dynamics in the molecule NO2 is presented. This experiment is based on using the channel-resolved above-threshold ionisation technique as a multi-dimensional probe of nonadiabatic dynamics in polayatomics molecules. The complex roles of one-photon excitation, multiphoton excitation to higher-lying neutral states, multi-channel neutral and ionic dissociation are examined and complications of using strong-fields to probe photochemistry are outlined. Chapter 3 deals with a VUV/UV time-resolved photoelectron velocity map imaging (VMI) study of Rydberg-valence mixing in high-lying excited states of acetone (CH3-CO-CH3). Details are provided of the implementation of a high-flux femtosecond VUV source based on non-collinear four-wave mixing. The VMI results show evidence of non-adiabatic evolution from the initially prepared 3dyz electronic state to the lower-lying 3p/3s states and subsequent relaxation of these Rydberg states to the pipi* electronic state on a few hundred femtosecond timescale. Chapters 4 and 5 deal with energy-resolved synchrotron radiation studies of the core-level ionisation, excitation and Auger decay in CH3I. Detailed assignments and analysis of the angular distributions connected with the I 4s, 4p and 3d orbitals are provided. These studies are a necessary precursor to aid in the interpretation of time-resolved data obtained using recently developed laboratory and facility based soft X-ray sources. Finally, Chapter 6 provides a summary of the different opportunities offered by performing time-resolved photoelectron spectroscopy in various wavelength regimes and provides an outlook for future work utilising novel ultrafast light sources.