Parametric excitation is a fundamental feature of dynamical systems arising across the applied sciences. In this thesis we study the structure of parametric res- onance and its in uence of the global nonlinear dynamics in a number of oscillating systems which arise in engineering contexts. The parametrically excited Helmholtz oscillator and the elliptically excited pen- dulum are two systems where the interaction of regular and parametric excitation are important for a complete understanding of the dynamics. We examine the resonance structure of the Helmholtz oscillator and use the Melnikov function to demonstrate the e ect that the parametric excitation has on the nonlinear dynam- ics. The estimates produced in this analysis are then compared to a numerical study of the engineering integrity. For the elliptically excited pendulum we discuss the quantitative e ects of introducing ellipticity to the pro le of excitation. We go on to examine the e ect of periodic time varying mass in the Helmholtz oscillator and demonstrate that the resonance structure exhibits the phenomenon of coexistence. The evolution of the systems engineering integrity is examined and compared to the purely parametrically excited case. Finally we examine a system incorporating two pendulums on a rigid rig modelled by two linear springs. The parametric resonance in this case is mapped using numerical Floquet theory and the structure of the linear resonance is shown to organise solution space for the nonlinear system.