In this book, two of the primary sources of flavonoids, i.e. 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 pretreatment and its combination with separation techniques, is also included.