Rubble mound coastal structures typically contain granular filters in one or more layers. These filters are normally geometrically tight (to prevent material washout), often difficult to realize in the field, and expensive. An alternative is a geometrically open filter. A geometrically open filter has a large ratio of the size of toplayer material and underlayer material and is designed in such a way that it fulfills its filter functions with only minimal base material loss or settlement. Potential applications of open filters include bed protections and toe & slope configurations of coastal structures. Proper guidelines on the design of open filters under wave loading could lead to significant cost and material savings, and to a more practical application of filters in the field. Physical model tests were conducted in a wave flume. These tests focussed on granular open filters on a 1:7 sand slope under wave loading. The analysis of the tests has led to guidelines on the amount of erosion of the sand underneath a granular filter and of the sand accretion within a granular filter.