Aircraft jet engines must operate in a stable manner at all times. One source of instability is compressor stall. Stall problems can be reduced by machining cavities into the compressor casing adjacent to the rotor blades. This ‘casing treatment’ is the focus of the present work. Two treatment configurations are tested: circumferential grooves cut into the casing above the rotor blades, and axial slots cut into the casing adjacent to the rotor blade leading edges. The performance of a single casing groove is measured at different axial locations over the blade tips. For the first time, it is shown that there are two locations where compressor stability is maximised; near the leading edge and near mid-chord. The interaction between the groove and the compressor flow field is then studied. It is found that when located near the leading edge, the groove has a strong interaction with the near-casing flow and tip leakage vortex, but when located near mid-chord, the interaction is more subtle and less damaging to efficiency. Since the groove works well in both locations, it is concluded that manipulating the tip leakage vortex is not critical for improving compressor stability. Different groove numbers and cross-sections are then tested. For multiple grooves, the effi- ciency reduction is the sum of the constituent grooves, while the stall margin improvement is less than this sum. A simple square-section groove is found difficult to improve on, although in certain circumstances, a new ‘intermittent groove’ geometry is beneficial. The performance of axial slots is then investigated. Different slot shapes are tested and the results added to previous work to suggest an optimum slot geometry. A computational flow study shows that large variations in blade loading occur as the blades pass the slots, which could cause noise and vibration. It is found that while the flow inside the slot is principally a quasi-steady recirculation, the interaction between the slots and blades is highly unsteady, and this unsteadiness should not be neglected in design. In general, it is found that casing treatments that generate large stability improvements cause large efficiency losses. It is shown for the first time that the performance of casing grooves can be seriously reduced by changes in the stall inception mechanism. Maximum performance is achieved when the treated compressor stalls with a spike inception. Models from the literature are tested, but do not predict the stall inception mechanism well, which makes predicting the performance of casing grooves in a given compressor hard. Finally, it is shown that designing the blades and casing treatment as a unit may improve compressor performance.