The fate of As in geothermal systems is controlled by the interaction of several geochemical and biological processes, such as dissolution, precipitation, dilution, volatilization, sorption, biomineralization, and biocatalysis. Although these interactions remain poorly understood, the distinctive micro- and molecular-scale fingerprints of processes occurring at the solid-water interface are shedding new light on these complex systems. The microscopic and molecular speciation of As in geothermal deposits offers valuable information about its chemical evolution, the processes in which it is involved, and the reactivity of the solid materials formed. The amorphous and hydrous nature of most hot spring minerals renders the application of conventional identification methods based on X-ray or electron diffraction challenging. This chapter presents a brief review of As in geothermal deposits and some of our own recent results on hot spring deposits in the El Tatio geothermal field. We used X-ray absorption spectroscopy to investigate the speciation of As in sinter material and in biological mats. Our results suggest that Fe oxyhydroxides play an important role in controlling the concentration of As in both solid media. Although it is recognized that microbial populations in geothermal systems may catalyze the rapid oxidation of As(III) to As(V), it is still unclear whether microbial catalysis and/or biomineralization control the formation of Fe oxyhydroxides. Understanding the association between As and Fe oxyhydroxides in geothermal systems is a key factor for evaluating the environmental impacts of geothermal energy exploitation as well as designing effective remediation approaches.