Cornelis Nicolaas Maria "Cees" Renckens (born 29 May 1946) is a Dutch Doctor of Medicine, gynaecologist, and a well-known skeptical activist against quackery.

From 1988 to 2011,[2] he was president of the Vereniging tegen de Kwakzalverij (VtdK: Dutch Society Against Quackery), which has been actively opposed to all non-science-based medicine since its foundation in 1881.

In 1991, the Society's board offered Wim de Bie, of the television comedy duo Van Kooten en De Bie, an honorary membership in the society for his personification of "Berendien uut Wisp", a fictional quack offering various dubious herbal treatments without expertise; this was a clear pastiche of a real televised controversial herbalist known as Klazien uut Zalk.

Instead of accepting the honorary membership, Van Kooten and De Bie invited Renckens to join them on their weekly satirical programme for an interview, in which he explained that he was filing a (fictional) complaint against Berendien, and gave serious reasons for opposing quackery.

[1]: 88, 96 On 12 October 2004, Renckens graduated from the University of Amsterdam with a Doctor in Medical Sciences with his thesis Dwaalwegen in de geneeskunde ("Misguided Paths in Medicine").

[6] Former emeritus professor clinical biochemistry Piet Borst expressed the opinion that Renckens deserves respect and praise for his uncompromising and determined combat against unscientific forms of care by physicians.