In this paper, I summarized various studies done by myself and my co-workers and related works by other researchers in terms of speciation and chemical-state of trace elements in various phases including aqueous phase, solid phase, and the solid-water interface to understand various geochemical reactions. In the aqueous phase, speciation can be easily conducted for various elements based on the stability constants reported for dissolved complexes if we can assume the equilibrium condition in the system. However, the stability constants for the species complexed with humic substances are not reliable yet especially in high salinity water. Speciation of rare earth elements in natural waters are discussed in this paper based on the reported data of the stability constants and REE patterns. In addition, systematics of predominant dissolved species of various elements is also discussed when considering hydrolysis, carbonate, and humate complexes as the dissolved species in natural aquifer. The hydrolysis is important for cations having relatively larger ion potential (= z/r), while carbonate and humate formation are for the cations with smaller ion potential. For the ion at the solid-water interface, there is a limited number of methods that can be applied to study the chemical-state. Laser-induced fluorescence spectroscopy was applied to Eu (III) sorbed on various media such as ion-exchange resin, clay minerals, and silica in the absence and presence of humic acid. The sorbed species estimated by the spectroscopic method was consistent with the features found in REE distribution patterns such as the slope of REE pattern and the appearance of the tetrad effect between aqueous phase and montmorillonite surface. Compared with the trace elements in aqueous phase and at the solid-water interface, it is possible to obtain direct information from trace elements in solid phase such as rocks and soils by sensitive spectroscopic method like XAFS. Examples of our application of XAFS to Ce and As in rocks and soils were given in this article. In particular for Ce, we can obtain various information from the comparison of the Ce (IV)/Ce (III) ratio and the degree of Ce anomaly found in REE patterns. We have applied similar technique to many trace elements such as Eu, Os, Re, I and S, details of which will be given elsewhere.