Metabolites are small organic molecules produced during metabolism that play key roles in energy production, growth, and cellular signaling. They are broadly classified into primary and secondary metabolites, both vital for survival, adaptation, and biotechnology applications. Metabolites are the intermediate molecules formed during the numerous metabolic reactions that occur within living cells. They are essential for maintaining life, as they participate in growth, development, energy production, and the regulation of cellular activities. Metabolites are the intermediate products of metabolic reactions catalyzed by various enzymes that naturally occur within cells. Metabolites, an international, peer-reviewed Open Access journal. Metabolites represent the end products, intermediates, and starting materials of cellular metabolic pathways, acting as a direct functional readout of the cell’s activity. They include compounds such as sugars, amino acids, lipids, and vitamins, that are constantly cycled through the body’s systems. Metabolites have various functions, including fuel, structure, signaling, stimulatory and inhibitory effects on enzymes, catalytic activity of their own (usually as a cofactor to an enzyme), defense, and interactions with other organisms (e.g. pigments, odorants, and pheromones). Metabolites are small molecules that supply the cell with energy, structural constituents and the materials to enable the synthesis of other macromolecules such as DNA or proteins. Metabolites is an international, peer-reviewed, open access journal of metabolism and metabolomics, published monthly online by MDPI. Living systems are defined not only by their genes or proteins, but by the countless small molecules that constantly flow through cells, tissues, and biofluids. These molecules—collectively known as metabolites—represent the most immediate and dynamic readout of biological activity. Metabolites can be defined as the molecules involved in metabolic pathways, which are linked chemical reactions within cells. They serve as substrates for subsequent reactions and are often categorized into classes such as amino acids, lipids, carbohydrates, nucleotides, and coenzymes.