Cement-Bentonite (CB) walls are low permeability vertical cut-off barriers that are mainly used to prevent groundwater flow and isolate contaminated areas. The hydraulic performance of the CB walls depends on the flow rate (discharge) through the wall. Therefore, the walls must achieve very low hydraulic conductivity. Poor in-situ hydraulic performance of these walls due to construction and post-construction defects may lead to surface settlements, groundwater contamination, and instability of the construction. This thesis investigates the range of in-situ hydraulic conductivity values of CB walls installed within the Netherlands. To achieve this, pump data for the various projects (Westerschelde Tunnel, A2 Motorway at Best, Motorway A4 Delft-Schiedam, Griftpark Utrecht, and Richard Hageman Akwadukt) were analyzed to determine the discharge through the walls. The in-situ hydraulic conductivity of the walls was calculated by Darcy’s law using groundwater level data and dimensions of the walls. The CB walls must be "keyed" into an underlying low permeable layer (aquitard), so that seepage of water through the aquitard is prevented. It was difficult to compute the in-situ hydraulic conductivity of the CB walls for Motorway A4 Delft-Schiedam and Griftpark Utrecht. The reason was that the walls in these projects were embedded in a permeable aquitard, which affected the total discharge value. The calculated in-situ hydraulic conductivity values were compared with the required hydraulic conductivity values and laboratory test results. In most of the cases, the in-situ hydraulic conductivity values of the walls were larger than the values acquired from the laboratory samples. The reason is that the estimation of the hydraulic conductivity of the CB walls from laboratory tests was based on small sample areas. This remains unreliable to evaluate the field performance of the CB walls. The average hydraulic conductivity of the CB walls at Richard Hageman Akwadukt was required to be less than or equal to 1·10-9 m/s. The calculated hydraulic conductivity values of the CB walls for the northern and southern polders were approximately 2.5·10-9 m/s and 2·10-8 m/s, respectively. This study also shows that the hydraulic performance of the CB walls is mainly affected by insufficient connections between the panels. Also, the impact of the defects on the discharge through the wall has been investigated by developing a threedimensional numerical groundwater flow model using MODFLOW. Simulations were performed for different types of defects, such as fully and partially penetrating windows, insufficient embedment, and connection between the panels of the CB walls. The results of the simulations indicated that fully penetrating windows have more impact on the discharge through the wall compared to partially penetrating windows. Partially penetrating windows with a hydraulic conductivity lower than 1·10-6 m/s are often insignificant. The size of a fully penetrating window also plays an important role. The difference in discharge (ΔQ) was calculated by subtracting the discharge through a perfect wall and imperfect wall (Qimperfect - Qperfect). The (ΔQ) through a CB wall with a window whose area is equal to 1 m2 can increase by a factor of 10 compared to a window whose area is equal to 0.1 m2. The location of the defects has no impact on the discharge through the wall. The simulations also show that the flow rate is unaffected by the embedment depth of the wall, but it must be ensured that the wall is in direct contact with the aquitard (low permeability soil layer). The vertical deviation of panels also affects the discharge through the wall. The MODFLOW results can be used to get an indication of the increase in discharge caused by various types of defects. From this, the possible defects for a particular project can be determined.