Molecular devices made of nucleic acids show great potential for applications ranging from bio-sensing to intelligent nanomedicine. They allow computation to be performed at the molecular scale, while also interfacing directly with the molecular components of living systems. They form structures that are stable inside cells, and their interactions can be precisely controlled by modifying their nucleotide sequences. However, designing correct and robust nucleic acid devices is a major challenge, due to high system complexity and the potential for unwanted interference between molecules in the system. To help address these challenges we have developed the DNA Strand Displacement (DSD) tool, a programming language for designing and simulating computational devices made of DNA. The language uses DNA strand displacement as the main computational mechanism, which allows devices to be designed solely in terms of nucleic acids. DSD is a first step towards the development of design and analysis tools for DNA strand displacement, and complements the emergence of novel implementation strategies for DNA computing.