High-Entropy Alloys (HEAs) are a new class of metallic materials based on the combination of multiple principal elements, often with the intention to form a concentrated solid solution. It has been suggested that these solid solution phases could offer enhanced mechanical properties, due to fluctuations in their local atomic environments. However, at present, the thermodynamic stability of the solid solution phases which form in HEAs is poorly understood. This study aims to improve our understanding of phase stability in HEAs and investigate possible differences in the mechanical behaviour of dilute and concentrated solid solutions. Firstly, a systematic series of experiments is presented, which establish the effect of Co and Fe on the phase equilibria of the widely studied CrMnFeCoNi system. Both Co and Fe were found to stabilise the A1 matrix relative to the A2 and D8b phases at elevated temperatures but did not prevent the formation of ordered phases at 500°C. Alongside literature data, the results show that stable single-phase alloys are rare in the CrMnFeCoNi system. Secondly, a systematic assessment of the AlxCrFeCoNi alloy system showed that Al promotes the formation of B2, D8b and A2 phases, and improved our understanding of phase stability in this system, which has strong potential for developing alloys with desirable mechanical properties. In each study, experimental observations were used to test the fidelity of thermodynamic predictions from the latest CALPHAD databases. Whilst the predictions generally provided a close approximation of the observed phase equilibria, inaccuracies were common, especially at lower temperatures. Phase stability investigations also served to identify thermodynamically stable alloys, suitable for the investigation of the Portevin-Le Chatelier (PLC) effect - a discontinuous yielding phenomenon widely attributed to the repeated pinning of dislocations by mobile solute atoms over certain regimes of temperature and strain-rate. Under mechanical testing, the effect is generally associated with serrations in the materials flow curve and the localisation of strain into discrete bands. Serrated flow has been observed in several HEAs but the associated strain localisation behaviour has not yet been reported, so it is uncertain whether this behaviour is similar to the PLC effect in other alloy systems. Thus, a series of alloys based on the stable equiatomic quaternary alloy, CrFeCoNi, were selected to investigate the effect of compositional complexity on the PLC effect. Tensile testing, combined with digital image correlation, enabled simultaneous recording of the materials stress and local strain response across a range of temperatures. The results showed close similarities between the alloys at each temperature, indicating that compositional complexity did not have a dominant influence on the PLC effect. This suggested that the dislocation-solute interactions in the complex, concentrated solid solutions were not substantially different from those of more dilute solutions. Calorimetric investigations also revealed evidence of a varying degree of short-range order among the alloys, the influence of which warrants further investigation.