Researchers and engineers using metallic iron (Fe0) for water treatment need a tutorial review on the operating mode of the Fe0/H2O system. There are few review articles attempting to present systematic information to guide proper material selection and application conditions. However, they are full of conflicting reports. This review seeks to: (i) Summarize the state-of-the-art knowledge on the remediation Fe0/H2O system, (ii) discuss relevant contaminant removal mechanisms, and (iii) provide solutions for practical engineering application of Fe0-based systems for water treatment. Specifically, the following aspects are summarized and discussed in detail: (i) Fe0 intrinsic reactivity and material selection, (ii) main abiotic contaminant removal mechanisms, and (iii) relevance of biological and bio-chemical processes in the Fe0/H2O system. In addition, challenges for the design of the next generation Fe0/H2O systems are discussed. This paper serves as a handout to enable better practical engineering applications for environmental remediation using Fe0.