Decision-making is a fundamental process of human thinking and behavior. In engineering design, decision-making is studied from two different points of view: users and designers. User focused design studies tend to investigate ways to better inform the design process through the elicitation of preferences or information. Designer studies are broad in nature, but usually attempt to illustrate and understand some aspect of designer behavior, such as ideation, fixation, or collaboration. Despite their power, both qualitative and quantitative research methods are ultimately limited by the fact that they rely on direct input from the research participants themselves. This can be problematic, as individuals may not be able to accurately represent what they are truly thinking, feeling, or desiring at the time of the decision. A fundamental goal in both user- and designer-focused studies is to understand how the mind works while individuals are making decisions. This dissertation addresses these issues through the use of complementary behavioral and neuroimaging experiments, uncovering insights into how the mind processes design-related decision-making and the implications of those processes. To examine user decision-making, a visual conjoint analysis (preference modeling approach) was utilized for sustainable preference judgments. Here, a novel preference-modeling framework was employed, allowing for the real time calculation of dependent environmental impact metrics during individual choice decisions. However, in difficult moral and emotional decision-making scenarios, such as those involving sustainability, traditional methods of uncovering user preferences have proven to be inconclusive. To overcome these shortcomings, a neuroimaging approach was used. Specifically, study participants completed preference judgments for sustainable products inside of a functional magnetic resonance imaging (fMRI) scanner. Results indicated that theory of mind and moral reasoning processes occur during product evaluations involving sustainability. Designer decision-making was explored using an analogical reasoning and concept development experiment. First, a crowdsourcing method was used to obtain meaningful analogical stimuli, which were validated using a behavioral experiment. Following this, fMRI was used to uncover the neural mechanisms associated with analogical reasoning in design. Results demonstrated that analogies generally benefit designers; particularly after significant time on idea generation has taken place. Neuroimaging data helped to show two distinct brain activation networks based upon reasoning with and without analogies. We term these fixation driven external search and analogically driven internal search.. Fixation driven external search shows designers during impasse, as increased activation in brain regions associated with visual processing causes them to direct attention outward in search of inspiration. Conversely, during analogically driven internal search, significant areas of activation are observed in bilateral temporal and left parietal regions of the brain. These brain regions are significant, as prior research has linked them to semantic word-processing, directing attention to memory retrieval, and insight during problem solving. It is during analogically driven internal search that brain activity shows the most effective periods of ideation by participants.