Metal halide perovskites have garnered a great deal of interest over the last few years due to their impressive – and rapidly increasing – performance as materials for solar cells. In particular, mixed-halide perovskites are essential for use in all-perovskite or perovskite-silicon tandem solar cells due to their tunable bandgap. However, one major obstacle to their use is the compositional instability some mixed-halide perovskites experience under illumination or charge-carrier injection, during which the perovskite material demixes into regions of differing halide content. Such segregation of halide ions adversely affects the electronic properties of the material and severely limits the prospects of mixed-halide perovskite technology. While, empirically, the crystallinity, stoichiometry, and optoelectronic properties of the perovskite – alongside other factors – are known to influence the halide segregation dynamics, the underlying mechanisms are still poorly understood. The work of this thesis elucidates many issues regarding the halide segregation phenomenon, and helps to explain the fundamental dynamics at the heart of this instability problem. The dependence of the halide segregation mechanism on the atmospheric environment surrounding the perovskite is investigated, revealing that encapsulation of the films with a thick layer of poly(methyl methacrylate) allows the segregation dynamics to be fully reversible and repeatable. From measurements of photon efficacy on halide ion separation, electronic trap states are revealed to play a pivotal role in the segregation mechanism. A predictive, empirical model is created to quantify this relationship, and is shown to match results in the literature. An investigation is conducted on full mixed-halide perovskite photovoltaic devices to study the impact of electric fields on both the dynamics of halide segregation and other mobile ionic behaviours. Three distinct defect species are identified in the perovskite, and their influence on the perovskite material is determined. Additionally, charge-carrier extraction from low-bandgap regions of perovskite suggests the existence of charge-carrier percolation pathways through grain boundaries. Finally, simultaneous, in situ X-ray diffraction and photoluminescence measurements are made utilising a home-built setup, and are used to study the role of stoichiometry on halide segregation in two different perovskite compositions. Fast ionic pathways – proposed to exist at grain boundaries – are found to strongly impact the ionic movement associated with halide segregation, resulting in extremely different compositional evolutions in high- and low-stability materials.