CallisonRTKL is a global architecture, planning and design firm formed from the October 2015 merger of Callison and RTKL Associates, both of which were subsidiaries of Arcadis NV, a global design, engineering and management consulting company based in the Zuidas, Amsterdam, Netherlands.
Callison was an international architecture firm based in Seattle, Washington. Callison was founded by Tony Callison in 1975 and grew to 900 employees around the world prior to its acquisition by Arcadis NV in 2014.
RTKL, instead, was a global architecture, planning and design firm founded in 1946 by Archibald Rogers and Francis Taliaferro and grew to be one of the largest architectural firms in the world prior to its acquisition by Arcadis NV in 2007.