When developers are faced with a design challenge, they often turn to the whiteboard. This is typical during the conceptual stages of software design, when no code is in existence yet. It may also happen when a significant code base has already been developed, for instance, to plan new functionality or discuss optimizing a key component. While software design at the whiteboard has long received an interest from the research community in terms of providing automated support through electronic sketching tools, the tools to date fall short in that each supports only a small subset of the design behaviors that software designers exhibit while working at the whiteboard. Indeed, the key insight motivating the research presented in this dissertation is that designers do not "operate in" or apply just a small handful of such design behaviors, but flexibly switch among many of them, all the time. As such, tools that only support a few design behaviors at the expense of others are inadequate to fully support the rich activity of software design at the whiteboard. In this dissertation, I present Calico, a distributed, collaborative sketch-based environment that supports a broad range of design behaviors. Calico offers four features: (1) scraps and connectors to manipulate content and build higher-order representations, (2) intentional interfaces to partition and organize the drawing space, (3) a palette to reuse content, and (4) a fading highlighter to make temporary marks on sketches. These features were designed based on an extensive study of the literature on software designers at work, and yield a minimally-invasive, yet rich environment for supporting design at the whiteboard that maintains the fluid and sketchy feel of sketch-based design. Calico was evaluated with designers in a controlled laboratory experiment, as well as in a longer-term in-the-field deployment, which included two software teams in industry and one research group. Findings show that, while there was a great amount of diversity in the design approaches by the different groups, the features do support and indeed are used for the design behaviors in actual work. Further, the designers reported that they sketched more than they would have on the whiteboard, built and refined their own types of representations, managed a growing number of sketches by organizing them into multi-page narratives, were more likely to return to old sketches, and switched between synchronous and asynchronous work during meetings. The contributions of this dissertation are four-fold: (1) The establishment of design behaviors as a necessary quality to support in informal design at the whiteboard. (2) A set of features to support informal software design at the interactive whiteboard. (3) An extensive evaluation of these features in a controlled laboratory setting and in the field with professional designers using Calico to perform their everyday work. (4) The construction of an extensible, sketch-based environment for early-phase software design. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]