Claudins are the main determinants of barrier properties of the tight junction. Many claudins have been shown to act by tightening the paracellular pathway, but several function as paracellular channels. While some depend on the endogenous claudin background of the analyzed cell line, for other claudins, a distinct charge‐selectivity has been shown. This paper portrays cation‐selective (claudin‐2, claudin‐10b, claudin‐15) and anion‐selective (claudin‐10a, claudin‐17) claudins and claudins with debatable channel properties (claudin‐4, claudin‐7, claudin‐16). It also describes molecular properties determining the observed charge‐selectivity and pore properties in general. In leaky tissues, they widely determine overall transport characteristics by providing paracellular ion‐selective pathways. In small intestine, claudin‐2 and claudin‐15 replace each other in the developing gut. In kidney proximal tubules, claudin‐2, claudin‐10, and claudin‐17 allow for paracellular reabsorption of sodium, chloride, and water.