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Bacteria contain secondary carriers for the uptake, exchange or efflux of C4-dicarboxylates. In aerobic bacteria, dicarboxylate transport (Dct)A carriers catalyze uptake of C4-dicarboxylates in a H+- or Na+-C4-dicarboxylate symport. Carriers of the dicarboxylate uptake (Dcu)AB family are used for electroneutral fumarate:succinate antiport which is required in anaerobic fumarate respiration. The DcuC carriers apparently function in succinate efflux during fermentation. The tripartite ATP-independent periplasmic (TRAP) transporter carriers are secondary uptake carriers requiring a periplasmic solute binding protein. For heterologous exchange of C4-dicarboxylates with other carboxylic acids (such as citrate:succinate by CitT) further types of carriers are used. The different families of C4-dicarboxylate carriers, the biochemistry of the transport reactions, and their metabolic functions are described. Many bacteria contain membraneous C4-dicarboxylate sensors which control the synthesis of enzymes for C4-dicarboxylate metabolism. The C4-dicarboxylate sensors DcuS, DctB, and DctS are histidine protein kinases and belong to different families of two-component systems. They contain periplasmic domains presumably involved in C4-dicarboxylate sensing. In DcuS the periplasmic domain seems to be essential for direct interaction with the C4-dicarboxylates. In signal perception by DctB, interaction of the C4-dicarboxylates with DctB and the DctA carrier plays an important role.