Today’s organizations deal with a significant complexity of their information systems (IS) architec-ture—a complex cobweb of heterogeneous IS with tight, mutual interrelations. With the constantly in-creasing number of IS along with the inherent complexity of the organizational context in which IS are embedded, organizations lose control of their IS architecture’s evolution. Through employing a se-quential mixed-methods research design, this study investigates the drivers and effects of IS architec-ture complexity. Based on the extant literature and on focus groups data, at the outset we develop a research model and derive its constitutive hypotheses. We subsequently test the research model follow-ing a partial least squares (PLS) approach to structural equation modelling (SEM) with survey re-sponses from 249 IT managers and architects. While differentiating structural and dynamic complexi-ty, this study confirms a high degree of integration, large size, high diversity, strong dynamics, and, in particular, inadequate planning as the main drivers of IS architecture complexity. Further, this study affirms the negative effect of IS architecture complexity on the efficiency, agility, comprehensibility, and predictability of the IS.