An object type that has puzzled archaeologists for a long time is clay balls. These are approximately 1 cm in size, usually round ceramic balls. They have been found during excavations since the early 1900s but have not yet received a satisfactory interpretation. Several proposals have been made but none of them gives a general explanation. The author has studied all presently known clay balls in Sweden, which number about 5000, distributed across about 50 prehistoric sites. These include settlements, graves and places of sacrifice. The features in which they are found are mainly hearths, layers of packed stones and graves. Several finds of potsherds and flint debris are found in combination with handicraft items.The common denominator of all clay ball finds seems to be transformation, transformation of material from one state to another. In some contexts, the transformation has to do with fertility, while in other cases the changes are of a more practical nature. Clay balls were probably a ritual symbol for the process of transformation and as such they were sacrificed in several different contexts. Possibly the symbolism has a female connection. It may be linked to an unclear kind of feminine cosmology. The use of clay balls has been dated to the BronzeAge and Early Iron Age, especially the Late Bronze Age and Pre-Roman Iron Age.