How do designers explore a design space to generate diverse solutions? This work focuses on the identification of design heuristics used in the ideation process in the domains of industrial design and engineering design. Design heuristics are cognitive strategies applied to a design problem to help designers create novel solutions. In a think-aloud protocol, engineers and industrial designers with varying levels of experience were asked to develop multiple concepts for a novel design problem. The results show evidence of frequent heuristic use, and that heuristics are effective in generating diverse, creative, and practical concepts, which, as a result, may stimulate higher-quality designs