Lean UX is a novel framework for bringing lean philosophy to User Experience design. Traditional approach to design often becomes the bottle-neck in agile software devel-opment, requiring heavy up-front work. Lean UX attempts to solve this by applying lean principles to the design work. The framework was originally developed by Jeff Gothelf and Joshua Seiden. Style Guides are a collection of guidelines, rules, standards and best practices. Their aim is to facilitate work by speeding up the design and implementation processes and making the products consistent at all levels throughout the processes. This Master of Science thesis examines how Lean UX framework can be applied in producing a style guide. The thesis focuses mainly on the practical application of Lean UX, with the style guide serving as the vehicle for applying the framework. The con-tributions of this thesis are the practical application of the Lean UX framework and the synthesis of some of the best practices such as a canvas approach to the design strategy and application of the Google HEART framework together with Lean UX. The thesis was written as a part of the UX team in the product development of Insta Response. Insta Response is a product family of mission critical emergency center ap-plications. Some of the selling points for the product family are the ease of use and a seamless experience throughout the products. To better achieve this, a style guide was needed. The Lean UX applied together with a UX Canvas and the HEART framework provided a solid foundation for taking the style guide from an idea to building and releasing the style guide. The canvas made the principles of Lean UX visible and concrete and al-lowed to perceive the relationships between the principles. The canvas also lent itself for integrating the HEART framework to the Lean UX process. During this work, two iterations of the Lean UX Build–Measure–Learn loops were run producing two different Minimum Viable Products. The hypotheses and metrics were adjusted between the iterations based on what was learned. The measured metrics indicated that the style guide was fairly successful in achieving its goals. However, some of the results indicate that another format for the style guide might be more efficient. In a wider perspective, the work conducted on the style guide formed a base for the UX team to adopt the Lean UX more widely with its practices.