The characteristics of animal waste composts produced in Japan were summarized. Animal waste composts are useful for supplying nutrients to plants and for improving the soil properties. The contents of nutrients in the composts were in the order of poultry ＞ swine ＞ cattle. Thus, the effectiveness as fertilizer followed the same order. The contents of heavy metals in the animal waste composts were studied. The content of copper was very high in the swine waste composts, and the content of zinc was also high in those from swine and poultry. Such high contents of heavy metals are caused by the addition of these elements to feeds. The objectives of composting were summarized. One of the objectives is to convert the raw animal wastes into products which are easy to handle and safe to health. Another objective is to convert the animal wastes into organic fertilizers which are safe for plant and soil. The maturing process of composting was characterized. Poultry wastes were more easily decomposed than cattle and swine wastes, and the decomposition rate of cattle wastes was almost similar to that of swine wastes. Maturity is an important factor related to the quality of the compost. The detection of nitrate by diphenylamine, the determination of cation-exchange capacity (CEC), and the germination test are recommended as methods of estimating the degree of maturity.