This paper inquires into the complexities of contemporary IT solutions based on a case study of the EU’s eCustoms initiatives using Manuel DeLanda’s Assemblage Theory. Technological innovations have enabled information infrastructures with dramatically increased number and heterogeneity of included components, and their dynamic and unexpected interactions. Unfortunately, lack of understanding of how the increasing complexity influences development initiatives is hampering effective information infrastructure development. Assemblage Theory can be seen as holistic synthesis of previous research streams seeking to explain how information infrastructures evolve in social contexts. Accordingly, in this paper it helps us getting a holistic grasp of the complexity of contemporary IT solutions and the “essence” of their dynamics. Through Assemblage Theory we explain how the European eCustoms information infrastructure has through a dialectic between stabilizing and destabilizing processes assumed its current shape - a result of decisions taken decades ago, path dependency, complexity, and accumulated emergence.