GridBox is an open platform for monitoring and active control of distribution grids. It is based on an innovative concept that comprehensively addresses the challenges DSOs will be exposed to in the context of increasing amount of decentralized and often fluctuating generation as well as the electrification of the heat and transportation sector. In this paper, we outline the principles of the GridBox concept, we describe its key elements in terms of hardware and software and we specify functionalities and applications. The practical implementation of the concept is illustrated by presenting an overview of the results from field tests in two different regions in Switzerland one in an urban grid area in the city of Zurich and one in a rural grid area in the canton of Bern. Results from the evaluation campaigns for state estimation and optimization algorithms are presented.