Textile materials and textile design are a part of countless products in our surroundings, as well as of diverse design fields and industries, with very different material traditions and working methods. Textile materials and industry have undergone many changes during recent decades, in terms of how and where textiles are produced, and what textiles can be and do; in much the same way, the design practices that textiles are involved in have also developed. What these diverse and evolving design contexts in which textiles are involved in have in common is that textile materials and textile design decisions somehow meet the rest of the design during a design process. The aim of this thesis is to add to our understanding of the relationship between textiles and products in the design process, and to explore the roles that textile design plays when designing textile products, the roles they can come to play when textiles become more complex and offer new means of functionality and expressiveness, for example through smart textile technology. This thesis presents two types of result: Firstly, descriptions of textile product design processes that highlight the wide range of roles that textiles can play in the textile product design processes of today, accentuate how textile materials and design decisions can influence both what can be designed and the design process, and describe some of the additional complexities that come with designing and designing with smart textiles. These examples are presented in the appended papers, and are the outcome of an observation of students who were designing textile products and collaborative, practice-based design research projects. Secondly, this thesis presents a theoretical framework which aims to offer a broad perspective on the relationship between textile design and the product design process, with the intention of opening up for reflection on how we design, and can design, with textiles. The framework focuses on how textile design decisions and textile materials participate in the process, and to what degree they influence the development of the design; this includes methods, questions, etc. that can be used to explore and define this dynamic. One of the main points of the framework is the importance of the textile influence in textile product design processes; the specific qualities of textiles as a design material - the considerations, possibilities, and challenges, which influence both the design of the product and the process of designing it. This includes not only the textiles in the final design, but also the textiles that, in other ways, feature in this process.