Hand-drawn sketches have traditionally been used to depict design ideas because they are quick to draw and can include as much or little detail as is required to convey the essence of the ideas. Computer tools are now an alternative and offer advantages for editing, storing and transmitting designs. However, designers consistently reject using current computer tools because these tools interrupt the creative process. Various studies have supported the designer's position, consistently showing that traditional tools produce more and better design ideas. This thesis describes the development and evaluation of a design-friendly computer tool that focuses specifically on the needs of the novice programmer who is designing user interfaces. From an extensive review of the literature on design, learning to programming and previous sketch tools we extracted the specifications for a tool that: compares favourably as a design medium with traditional tools such as the pen, paper and whiteboards, provides the editing and storage support expected of computer tools, helps students to gain a better understanding of programming problems and integrates seamlessly into a program development environment. Freeform, the tool we have developed to these specifications, has had two iterations of development and usability testing. This tool is unique in that it: is integrated into a commercial program development environment, uses a digital whiteboard for interaction and includes character recognition. Using Freeform, students can both quickly hand-draw user interface designs and interact with the design while it is still rendered as a sketch. When satisfied with the design the student invokes the recognition engine. The sketch is then overlaid with recognition data. Any incorrect interpretations can be altered by the student. The student can then instruct Freeform to create the formal user interface in the program development environment. The translation of sketch glyphs to user interface widgets is achieved by parsing the sketch and recognition data with transformation rules. We have conducted two evaluation studies using Freeform. The first study directly compared Freeform to a traditional alternative. We found that, although the design quality was similar, the students believed that when they were using Freeform they both understood the problem better and enjoyed the experience more. We noted during this study that the interactive checking available in Freeform prompted more changes to the designs than the static checking done on a standard whiteboard. In the second study, we asked students to check designs rendered as both sketches in Freeform and as formal diagrams in a user interface builder. The students made significantly more revisions to the Freeform sketches and therefore produced better designs from Freeform. The usability tests and evaluation studies we have conducted suggest that computer-based low-fidelity design tools: can compete favourably with traditional tools as a design medium, offer better support for editing and storage, and may have advantages for checking over both traditional sketch mediums and formal interactive computer designs.