Kaj Ulrik Linderstrøm-Lang (29 November 1896 – 25 May 1959) was a Danish protein scientist, who was the director of the Carlsberg Laboratory from 1939 until his death. His most notable scientific contributions were the development of sundry physical techniques to study protein structure and function (especially hydrogen–deuterium exchange), and his definitions of protein primary, secondary, tertiary and quaternary structure.