In this book, two of the primary sources of flavonoids, ie: plants and fruits (oranges to be specific) are comprehensively reviewed, along with their flavonoid content and antioxidant activity (including action against COVID-19) and toxicity. Besides, a general overview of the analytical techniques for flavonoids determination in their natural sources is provided. Finally, a detailed description of flow techniques and their evolution, together with their role in the automation of the sample pre-treatment and its combination with separation techniques, is also included.