The use of moonpools for the launch and retrieval of diving bells and remotely operated vehicles from a mother ship is well established, and the advantages of the moonpool approach are well known. The use of moonpools in Floating Production Systems is also becoming more common, as a means of allowing the marine riser to enter the vessel. The moonpool offers protection from wind and current forces and reduces the effects of angular motions of the vessel. The only problem with the moonpool concept is that the water column inside a badly designed moonpool may suffer from large and apparently unpredictable vertical surges, making operations through the moonpool hazardous, and in extreme cases threatening the seaworthiness of the vessel. The work described here shows how a moonpool design may be optimised for a particular vessel in order that such problems may be avoided. The dynamics of the problem are established, such that the water column oscillation and the forces on a subsea unit in the moonpool may be predicted. The effects of a variety of geometrical configurations are then studied, both mathematically and experimentally, in order to select configurations which allow large modifications of the moonpool response. A quantitative measure of moonpool performance is proposed, allowing individual designs to be ranked in terms of the long term expected downtime due to the moonpool. Finally, the understanding and results thus gained are integrated in order to produce a practical design procedure for a moonpool of any size, in any vessel, and in any operational area. A worked example based on a real design problem is presented in order to illustrate the practical application of the method.