Trace detection for most metals in ion chromatography is a major challenge because of the lack of exploitable physico-chemical characteristics in aqueous solution. This chapter focuses on the main approach for solving this problem, namely, the use of post-column reaction methods involving colorimetric, fluorometric and luminescent reagents. The characteristics and operating parameters of the most commonly used reagents are discussed in detail and their sensitivities and limits of detection compared. Attention is also paid to the key features of post-column reagent delivery systems, mixing and reaction coil construction and the various types evaluated. Of special interest are the latest developments in post-column reagent optimization using computer modeling of the main species involved and methods of improving the signal to noise ratios to obtain the lowest limits of detection. The reader will also find valuable advice and tips based on over 20 years experience by the authors in studying and operating post-column reactions.