This thesis examines trends in the academic and professional literature around immutable data and its relationship with declarative User Interfaces (UIs). Immutable data types are preferred by academic authors due to their increased safety, and commercial languages are increasing their support for them over time. More recently, declarative UIs are an exploding topic in industry, and these are related, although not as closely as one would expect. Declarative programming tries to focus on highlevel requirements, not low-level details. It is easier to do this if functions have no side effects, and immutable data is a guaranteed way of achieving this. To highlight this property, the declarative UI framework Flutter advertises “stateless widgets”, but their existence puts in highlights the lack of this property in most widgets. Consequently, we ask whether it is feasible to build a Graphical User Interface (GUI) toolkit using purely immutable data structures. To accomplish this objective, a purely immutable GUI toolkit is sketched and partially developed using Elm, a purely functional language in which all data structures are immutable. To understand the requirements of a GUI toolkit, we categorize and put in historical context, different design paradigms for UIs and relate them to core software-design principles. Leading toolkits allow developers to visualize and manage multiple views of their interfaces, including the view hierarchy, layout, interface to business logic, and focus management. By creating a concrete example, the research aims to provide insight into the limitations of utilizing purely immutable data within a GUI framework and suggests future work to mitigate these. / Thesis / Master of Science (MSc)