By using modular product structures, the external product variety required by customers can be realized with a minimum of internal variety of components and processes. Understanding the effects of this special product structuring strategy is essential in the early product planning phase to optimize the product structure for the entire value chain. For this purpose, an empirical study was conducted and is reported in this paper. Four real cases were analyzed to identify the effects of modular product structures considering effect occurrence and project-specific boundary conditions. The results are visualized in an extended impact model of modular product structures and traced back to possible measures of product structuring as well as essential target values in the company. These causal relationships serve product developers as decision support in planning and development of modular product structures.