In this review, we summarized the various results dealing with SLC26A9 expression at the tissue, cellular, and subcellular levels; its structure and function as anion channel or transporter; its interaction and functional relationships with CFTR; and its role as gene modifier, and tried to reconcile them in order to highlight the current understanding and the gap in knowledge regarding the contribution of SLC26A9 to human lung physiology and CF disease and treatment. This evidence concerns the gene CFTR and cystic fibrosis.