Sourdough is prepared from a mixture of flour and water by spontaneous fermentation or by using starter cultures. During fermentation, the dough constituents are modified by the action of the lactic acid bacteria (with or without yeasts). The sourdough fermentation has a number of beneficial effects that include prolonged shelf life, accelerated volume gain, delayed staling, improved sensory characteristics and good nutritional value of products. Phenolic compounds in sourdough are mostly present in bound form through ester bonds to arabinoxylan chain or through ether bonds to lignin. During cereal fermentation, lactic acid bacteria have to interact with inherent phenolic compounds. The latter are modified by the action of both endogenous and bacterial enzymes - feruloyl esterases, tannase, glycosyl hydrolases - thereby affecting their structure, bioactivity and bioavailability. Lactic acid bacteria have a high tolerance to antimicrobial phenolic acids found in wheat and rye; their resistance is partially dependent on their capacity to convert phenolic acids to metabolites with changed metabolic activity. Human health benefits associated with the consumption of sourdough phenolic compounds are, in part, depend on microbial conversion. Hydroxybenzoic and hydroxycinnamic acids are converted by decarboxylase to the corresponding phenol or vinyl derivatives. New metabolites are formed also by action of reductases which reduce hydroxycinnamic acids and their vinyl derivatives.