When instructional designers talk about what they do, they often discuss their outcomes rather than their process. Conjecturing during decision-making requires the instructional designer to build upon their prior knowledge of the situation and experiences and make assumptions based on available information to design an appropriate solution. This quasi-experimental design examined the types of conjecturing strategies used by instructional design students during design decision-making practices. Participants completed a design activity in real time while following a think-aloud protocol that was evaluated in terms of how they employed conjecture strategies. Findings indicate there is a significant correlation between an instructional designer's ability to conjecture and tolerate uncertainty. Additional findings support the use of reflection-in-action to manage uncertainty and explore multiple solutions during the design process.