Numerous trials were performed to optimise the assays to measure phytochemical contents that could link to the potent activities of polyphenolic antioxidants. I started my research in natural products in 2006 and collected ample scientific data to publish my first paper in Food Chemistry (Razali et al., 2008). Inspired and motivated by the first ISI-paper, our lab developed and established antioxidant assays that were continuously optimised and published in ISI-indexed journals (Razali et al., 2010, 2012, 2015a, 2015b and 2018). Thus, as a natural products scientist and nutritional biochemist, I write this book to provide updates on the optimisation of quite a number of established phytochemical assays. There are several in vitro methods for evaluating antioxidant activity. This book gives information regarding different methods that are used to measure the antioxidant defense system, as well as discussing the specific principles of their application. Hence, it will be a comprehensive ready reference for those who are interested in antioxidant study. As such it would benefit a broad readership including natural product researchers, pharmacologists and nutritional biochemists.