Factors that alter shells during the fossilisation process are reviewed, in order to explain the appearance of fossil and modern shells from the Dutch beaches and estuaries. Biological agents that transform the structure and appearance of shells include boring organisms, shell-crushing predators, and parasites. These organisms may influence shell appearance already during life of the mollusc. Hermit crabs living in empty gastropod shells accelerate shell destruction. Physical damage to shells has been overvalued in the past. Physical abrasion is only a common feature in the tidal zone. All shells found on the beach are transported. Such a transport of shells may be over large distances of hundreds of kilometres when attached to algae (particularly Himanthalia) or other floating matter, but for the major part shells found on the beach represent communities from the shallow coastal zone in the vicinity of the beach. Also, shells eroded from older deposits outcropping on the seabed in the vicinity may contribute substantially to the beach assemblages.