The increase in competition amongst companies that produce complex or large product portfolios has created a need to utilise modularity strategies not only to flexibly manage technical complexity in a costeffective manner but also for visual appearance. This research aims to understand how the visual appearance of products is affected by modular product development strategies. Specifically, the aim is to understand how such strategies induce constraints and generate possibilities for management of visual appearance in the design process. Five studies have been conducted during the course of this licentiate thesis. Two were conducted with professionals and students in design, while the remaining three are theoretical studies based on findings in the literature, theory building, and experimental research. The goal has been to investigate how designers work when they are put to the task of changing and developing the designs of complex products that are part of a portfolio. The challenge has been to study what suitable strategies exist that manage complex products and product brands, then investigate how these influence designers’ practices. The first study examined how coherence towards a product category influences the design of new products. The outcome of the study was a method to explore visual coherence and diversity in the appearance of a product category. The remaining four studies investigated how modularity, brand management and the redesign of product portfolios influence a design process. The second study described a design phenomenon known as aesthetic flexibility, which was further explored in studies three and five. The outcome from these studies was a proposal for four aesthetic flexibility strategies. The fourth study investigated in what way portfolio extension strategies found in brand management and design research are related, and how such strategies influence aesthetic flexibility. The results from study four were illustrated as a model. The main contribution of this work is the phenomenon of ‘aesthetic flexibility’, which helps understand the factors that influence designers when working with branded modular products. Understanding visual flexibility serves as a starting point in further investigations of how different development strategies affect the possibilities for visual product design. The findings of this work serve to illustrate and explain a complex and multi-facetted design phenomenon which many designers manage more or less intuitively today, thus advancing academics’, teachers’ and professional designers’ understanding of the field.