This thesis discusses the effect of an obstruction on the evolution of the Rayleigh-Taylor instability in a confined geometry at low Atwood numbers. Laboratory experiments are the principal method of investigation, though these data are supplemented with implicit large eddy simulations (ILES). The laboratory data are captured using an innovative laser scanning system which is able to simultaneously record density and velocity data in 3D. A new approach for calculating density data from laser induced fluorescence measurements is developed and demonstrated. The technique is used to improve the accuracy of the density measurement from laser induced fluorescence, by correcting for the damage to dye caused by the laser. The introduction of an obstacle at the height of the initial interface results in dramatic changes to the dynamics of mixing, even when this obstacle is only a few percent of the domain width. Two obstructed scenarios are considered. In both of these an obstruction is placed on the interface between an upper heavy layer and lower light layer. In the first case, a single horizontal opening connects the upper and lower layers. A bidirectional flow exchanges fluid through the opening, establishing a circulation cell in each layer. These cells exist quasi-steadily for long periods, constantly recirculating and mixing the fluid in each layer. This acts to increase the time required for mixing compared with the classical unobstructed case, but results in a more uniformly mixed final stratification. The second case has two horizontal openings, one either side of the obstruction. This results in markedly different dynamics. The flow through each of the openings switches back and forth between being bidirectional (as with the single opening case) and unidirectional, with unidirectional exchange reversing direction with a constant period. These results are consistent with the ILES data. For both of these cases a wide range of analytical techniques are used to connect the new obstructed dynamics with previously conducted research, such as calculating the molecular mixing fraction, energetics and mixing efficiency. A multistage mixing process is identified, unique for cases with an obstruction. For the single opening case a hierarchy of models are developed that accurately capture the density change of each layer for both the experimental and numerical data. The effect of changing the aspect ratio of the domain is investigated using ILES, from which different dynamical regimes are observed, discussed and analysed.