**1. Introduction**

For decades, it has been commonly accepted that plant compounds have a wide range of biological activities. They are secondary metabolites that have considerable pharmacological characteristics and play an important role in improving human health, and flavonoids are one of the substances that have been isolated. Flavonoids, which are responsible for the color and perfume of flowers, have long been known to be synthesized in specific locations, and there are presently around 6000 flavonoids that contribute to the colorful pigments of fruits, herbs, vegetables, and medicinal plants [1]. Flavonoids are hydroxylated phenolic substances known to be formed by plants in response to microbial infection and are a broad set of polyphenolic chemicals with a benzo-pyrone structure synthesized through the phenylpropanoid pathway [2–4]. Fruits, vegetables, cereals, bark, roots, stems, flowers, tea, and wine all contain it. The chemical properties of flavonoids are determined by their structural class, degree of hydroxylation, various substitutions and conjugations, and degree of polymerization [5].

Flavonoids and other phenylpropanoids are generated from phenylalanine in plants, including the subgroups of flavanones (e.g., flavanone, hesperetin, and naringenin), flavones (e.g., flavone, apigenin, and luteolin), isoflavones (e.g., daidzein, genistein, glycitein). The quantity of oxidation and pattern of substitution of the C ring change within flavonoid classes, whereas individual compounds within a class differ in the pattern of substitution of the A and B rings [6]. Flavonoids are regarded to have health-promoting effects as dietary components due to their strong antioxidant activity in both *in vivo* and *in vitro* systems [7, 8]. Flavonoids' functional hydroxyl groups influence their antioxidant activities by scavenging free radicals and/ or chelating metal ions [9, 10]. In addition to antioxidant capabilities, flavonoids have been shown to have antiviral, antibacterial, anti-inflammatory, vasodilatory, anticancer, and anti-ischemic activities. The metabolism of flavonoids is performed by one or more membrane-associated multienzyme complexes rather than free-floating "soluble" enzymes [11]. The primary enzymes involved in flavonoid metabolism are chalcone synthase (a key enzyme in the phenylpropanoid pathway).

Although the separation, identification, and quantification of constituents in complex plant extracts and most likely will be a challenging task, today a multiplicity of different separation techniques, specific stationary phases, and detectors are available, helping to achieve the desired selectivity, sensitivity, and speed for nearly any separation problem. The most prominent and popular technique in this area of research is liquid chromatography [12]. Chromatographic techniques contributed significantly to the area of natural products, especially regarding identification, separation, and characterization of bioactive compounds from plant sources [13]. Flavonoid metabolism is a strong supporter in disease treatment and prevention with chemicals and is an indispensable ingredient in many nutritional, pharmaceutical, and cosmetic applications. The extensive research of flavonoid metabolism in the genome and metabolism is a great technique to investigate how plants' unique metabolic pathways originated during evolution. The coupling of liquid chromatography (LC) with ultraviolet (UV) and/or electrospray ionization (ESI) mass spectrometric detection is a potent instrument for the identification of phenolics in plant extracts. This chapter focuses on the chemistry and metabolism of flavonoids, as well as the use of liquid chromatography in the study of plant flavonoids.
