When a volcano eruptes, its pyroclastics are deposited over the surface of the earth, so the depositional features of the pyroclastics tell us the history of volcanic activities. Therefore, if we wish to investigate the tephrochronology of pyroclastics which spread over the surface of the earth, the following works should be done.1) At first, we must classify the sorts of the pyroclastics which spread over the surface of the earth and study the characteristics of each pyroclastic. Then we must research their areal distribution and the sources of their eruptions.2) Next, we must determine the time of eruption of those pyroclastics, and in this case the following principles should be adopted as fundamental ideas.a) When natural trees grow on pyroclastic forming the surface soil, we can estimate the age of the pyroclastic by means of calculation of their annual ring.b) When pyroclastic falls on peat lands, peat begins to develop on the pyroclastic. As the annual rate of increase of the peat layer thickness is considered to be constant, so by determining the depth of the peat layer growing on the pyroclastic, we can estimate the time when the pyroclastic was deposited.c) When we find carbonized trees in a layer of pyroclastic, we can estimate the age of the pyroclastic by means of determining the 14C of those carbonized trees.d) When we find a prehistoric site in a layer of pyroclastic, we can estimate the age of the pyroclastic by determining the age of the prehistoric site by archaeologists.e) When we find obsidian stone implements made by aborigines in a layer of pyroclastic, we can estimate the age of the pyroclastic by determining the thickness of the hydrated surface layer which has been formed outside the obsidian.f) Moreover, if we can correlate the age which has been estimated by means of the above mentioned methods to ancient records of the volcanic activity, we may possibly estimte still more accurately the age of the pyroclastic.