This thesis investigated the use of electrorheological fluids in shock and vibration isolation. An electrorheological fluid (ER) is a suspension of semiconducting particles in a nonconducting carrier fluid. The novel feature of this fluid is its ability to change its rheological properties upon the application of an external electric field. To investigate the properties of activated ER fluids an experimental oscillatory electroviscometer was commissioned and the fluid tested. Conclusions drawn from the results were that the activated ER fluid changed from a viscous system, to one with a yield stress, elasticity and enhanced viscosity. These rheological changes were catalogued for a variety of electric field strengths, oscillatory frequencies and amplitudes. The fluid was also incorporated into a damper in an experimental single degree of freedom base excited system (SDOF). It was shown that the relative displacement between the base and the isolated mass could be significantly reduced on application of an electric field. The feasibility of a 'switched' semi-active vibration isolation scheme was also demonstrated with the SDOF apparatus. This system aimed to reduce isolated mass acceleration. Numerical analysis was carried out which involved successfully modelling the response of the SDOF system incorporating the ER damper. This model used a 'softening' spring element, a 'nonreversible' Coulomb friction characteristic and viscous damping to represent ER material behaviour. A theoretical comparison was also made between activated ER systems and systems with hardening spring characteristics, for steady state and transient responses. It was shown theoretically that the semi-active scheme described can give major improvements in isolation performance for frequencies much greater than resonance. The numerical integration of the models was carried out using a fourth order Runge-Kutta algorithm with the aid of a novel smoothing function to overcome problems associated with modelling friction phenomena.