Solar prominences are structures formed of cool, chromospheric plasma magneti- cally suspended in the solar corona for up to several solar rotations. Prominences are observed to migrate through quasi-equilibria until their host magnetic field loses equilibrium with its surroundings. It has traditionally been assumed that this ob- served plasma evolution is a consequence of evolution in the magnetic field alone. However, recent results indicate that this interpretation is incomplete. This thesis uses a combination of observations and modelling to quantify the different contri- butions of plasma and magnetic field to the stability of solar prominences. Firstly, the effect of prominence plasma on its host magnetic field was deter- mined using an observational case study. The optical thickness of the prominence was used to estimate column density and mass in the lead up to its eruption. An estimate of the ratio of plasma and magnetic forces indicates plasma processes can heavily influence the equilibrium. The study of mass was then expanded to a more general study using an ana- lytical model. The full equilibrium governing a general prominence was quantified and used to show that the effect of plasma on the structure is two-fold. Firstly, in- cluding plasma in the equilibrium stabilises the prominence, when comparing with the massless case, additional magnetic forces are therefore required to overcome the modified equilibrium. Secondly, removing plasma from a prominence in equi- librium can enable the magnetic field to become unstable and cause an eruption of the prominence. Finally, the fine-scale evolution of plasma within a prominence was studied using a combination of high-resolution observations and state-of-the-art models. The results suggest that the force-balance varies over the length of the prominence, hence these structures cannot be considered solely magnetically dominated. This work suggests that the relationship between prominence plasma and mag- netic field on both global- and fine-scales contributes more to the stability of promi- nences than previously believed.