To test the stability of sand filled geotextile tubes under wave attack, tests were performed in the Delta Flume of Deltares on a scale of approximately 1:2 - 1 :4. Wave conditions in the model varied up to 1.5 m significant wave height. Seven test series with different geometric conditions were tested. In four test series, single placed tubes with varying filling percentages and sizes were tested. One test series consisted of a single tube with a bar placed at the landward side to simulate a trench. Two other test series consisted of 2 tubes placed behind each other and a so-called 2-1 stack; two tubes placed behind each other with a third tube on top. During all test series the normative failure mechanism was sliding, although some deformation occurred at the test series with lower filling percentages. The sliding mechanism is studied intensively resulting in a dimensionless stability parameter. This parameter includes the significant wave height (Hs), the height (0), width (8) and relative density (Ll) of the geotextile tube, a reduction parameter to include wave energy transmission (x), the slope angle of the foundation of the geotextile tube (a) and the friction coefficient between the geotextile and the foundation of the geotextile tube (t). Sand migration inside the geotextile tube may decrease its stability when the filling degree is less than approximately 70 %