Medicinal and aromatic plants are increasingly recognised as valuable functional feed additives in modern animal production. Owing to their high content of phytochemical compounds, these plants have long been utilised in food and medicine, and are now considered promising alternatives to synthetic feed additives and antibiotic growth promoters. Plant metabolism is generally classified into primary and secondary pathways; while primary metabolites are indispensable for vital plant functions, secondary metabolites—referred to as phytochemicals—are responsible for a wide range of bioactivities. Major groups of phytochemicals include phenolic compounds, terpenes, nitrogenous compounds, and sulphur-containing metabolites, all of which exhibit antimicrobial, antioxidant, anti-inflammatory, and immunomodulatory effects. When incorporated into animal diets, these bioactive compounds can enhance feed efficiency, support growth, strengthen the immune system, and improve overall health and product quality. Furthermore, their use contributes to more sustainable and environmentally friendly livestock systems by reducing reliance on synthetic additives. However, the beneficial effects of phytochemicals are dose-dependent, and excessive levels may cause adverse outcomes. A deeper understanding of their mechanisms of action and optimal inclusion levels is therefore essential to fully exploit the potential of medicinal and aromatic plants as functional feed additives.