Abstract Reserve estimation is a process commencing with sampling and culminating in the issue of a reserve statement. The tin, gold and diamonds which occur as marine placers on the continental shelf are low grade with a very erratic mineral distribution. Sampling with a variety of special equipment determines the extent and grade of the mineralization. The size of sample is important, especially for diamonds. Together with the sampling density, it determines the accuracy of the grade estimate. Bulk sampling is a confirmatory process. The choice of mining equipment, either new or developed from onshore activities, dictates the unit costs, the cutoff grade, the selectivity of extraction, and thus the size of blocks which comprise the mining plan and the reserves. A variety of estimation methods of different reliability are available, the most advanced being kriging. Dilution and losses during mining must be taken into account in estimation, and a grade-volume curve should be constructed. Diamond deposit estimation also involves an extra variable, the dollar value of the stones. During and after mining essential feedback on the accuracy of an estimation maybe given by the R/E factor and by a mineral balance. Reserve estimates are required for several reasons and may be subject to public disclosure. The use of the correct definitions is therefore important. Introduction The marine minerals under consideration are those of continental shelf deposits of tin, precious metals (gold and platinum), and diamonds, presently being mined in water depths of up to 200rn. Together with some industrial minerals, they are generally referred to as heavy minerals because of their relatively high specific gravities. They occur in very low concentrations in usually unconsolidated seabed sediments comprising marine placer deposits. Such deposits combine the worst of all problems in the mining industry. They are placers, recognized by those with the appropriate experience, to be the most difficult deposit type to evaluate. They are erroneously thought by some to be simple. Marine placers are obviously underwater and cannot easily be examined directly. The mining method may be uncertain, unknown, or involve entirely new technology. The first and most difficult stage of bringing a marine mineral deposit to production is the evaluation and the choice of mining method. Development of the two inter-woven subjects progresses in an iterative manner. After the deposit's initial discovery its evaluation comprises sampling, bulk sampling, and possibly trial mining. These stages ideally lead to the estimation and delineation of a reserve with a defined mining method for the deposit's exploitation in a financially viable manner. Methods have been developed onshore for the successful evaluation of their terrestrial equivalents. These have usually been derived as a result of many years of experience with the onshore deposits in question. Although no theoretical rationalization may exist' the methods have been successfully developed and proved by industry trial and error. Despite uncertainty as to how that methodology may he applied offshore it has at least provided a starting pointz, with modification, for the new marine environment.