Robustness is an important aspect of complex engineered systems. However, ambiguity in its definition can generate uncertainty among engineers about how to be sure that it is accounted for in their design. In this paper, robustness is defined as a property that allows a system to maintain its functions against anticipated internal and external perturbations. The important aspect of this definition is the focus of robustness on a per-function basis. A modified utility-based design approach is presented that provides a step by step implementation to help engineers ensure their designed systems meet their preferences and contain robust characteristics. The approach focuses on generating functional models of the proposed system. The functional models provide designers with insight into which internal and external perturbations should be included within the system model. An example case study is included to illustrate the steps of the modified utility-based design approach. The case study examines the entry, descent and landing of a Mars rover. The system and subsystems are modeled using black box models and EMS (Energy, Material, Signal) function structures. This allows the relevant internal and external perturbations to be modeled in the system model. The case study shows how using a utility-based analysis can produce a robust system. Copyright © 2013 by ASME.