In October 2015, Callison was formally merged with another Arcadis subsidiary, RTKL Associates, to form CallisonRTKL headquartered in Baltimore, Maryland.

[1] Callison planned and designed retail, hospitality, mixed-use, workplace, corporate and healthcare projects worldwide.

From its founding in Seattle in 1975, the company grew to over 900 employees worldwide, with offices across the globe: Seattle, Los Angeles, Mexico City, Dallas, Scottsdale, New York City, London, Dubai, Beijing, Shanghai and Guangzhou.

In August 2014, Callison announced it would be acquired by Arcadis NV, a global design, engineering and management consulting firm based in Amsterdam.

[3] In October 2015, the two firms were formally merged to form CallisonRTKL headquartered in Baltimore, Maryland.