Incubation, which involves putting the problem aside temporarily and performing an unrelated activity, has been widely shown to enhance creativity. It has also been proposed that incubation may facilitate design ideation, though relatively few studies have empirically examined this. As such, there is a lack of knowledge regarding the role of incubation in a design context, including the key psychological factors that may determine the effectiveness of incubation. One possible influencing factor is the mood of the designer during incubation. It is well known that creative cognition is influenced by mood, with positive moods in particular being linked with increased cognitive flexibility and novel thinking. Previous research has also indicated that positive mood during incubation can improve subsequent divergent thinking, although it is not clear whether similar effects occur in a design context. Against this background, this thesis aimed to assess whether incubation, in general, would facilitate subsequent design ideation. In addition, it aimed to examine whether positive moods during incubation would be particularly effective in enhancing ideation. A preliminary study was firstly conducted to establish the cognitive processes associated with the generation of novel design concepts and identify which, if any, of these processes were known to be influenced by mood. To this end, 101 product design engineering students performed a series of open-ended ideation tasks as well as a range of psychological tests that assess different cognitive processes, including measures of associative processing, executive function, mental imagery, and intelligence. Correlation analyses indicated significant positive relationships between idea novelty (assessed by 3 independent raters) and scores on associative flexibility, verbal fluency and fluid intelligence. In addition, a significant negative correlation was observed between inhibition and novelty. Finally, a regression model with the cognitive test scores showing significant correlation as predictors was found to be explain 26% variance in ideation novelty in the sample. Importantly, several of the cognitive processes highlighted as key to the generation of novel concepts (associative flexibility, verbal fluency and disinhibition) have been shown by previous research to be enhanced by positive mood. This suggested that positive moods during incubation, through stimulating these processes, may facilitate subsequent design ideation. This was formally tested in study 2, in which a group of product design engineering students (n=72) were randomly assigned to one of four conditions. Each participant performed the same ideation task but with: an incubation period with a happy mood induction, or incubation with sad mood induction, or incubation where no mood was induced or no incubation period and no induced mood. The happy and sad mood induction used in the experiment involved exposure to mood appropriate music and text. The neutral group were also exposed to music and text during the incubation period but this was not designed to induce any specific mood. The results suggested that firstly, there was no evidence of a general (i.e. non-mood related) incubation effect in that the control group and the neutral group showed no significant differences in terms of fluency or novelty of ideas generated. Secondly, there were no significant differences in ideation performance between the three incubation groups (i.e., happy, sad, neutral incubation), showing no support for the hypothesis that positive moods during incubation period would have a particularly beneficial impact on performance. Finally, while there was no evidence of an incubation effect, the results did suggest that in general, participants generated more novel ideas in the second half of the ideation task. This is consistent with a substantial body of evidence to suggest that people become more creative with increasing time on task. The findings of study 2 were inconsistent the wider creativity literature, where mood and incubation have been widely demonstrated to be key factors of influence on creative performance. However, much of the evidence for this is based on studies using standardised creativity tasks such as divergent thinking and insight tasks. Thus, a possible explanation of the findings was that incubation and mood simply have reduced influence in more domain specific, applied creativity settings. In general, incubation may interfere with some designers' established routines and preferences regarding if and when to take a break. In terms of mood during incubation, previous work has indicated that mood may exert counter-active effects on more applied creativity tasks, because while it enhances novel thinking and cognitive flexibility, it impairs the more constrained and analytical processes that are required for generating practical solutions - the result being no overall impact on ideation performance. The above arguments could be explored further by examining the effects of the mood-based incubation paradigm in a more domain-general creativity context. If incubation effects (either general or in relation to mood) were observed under such circumstances, this would give support to the notion that incubation and mood have more facilitative effects in general-creativity settings, with reduced influence in more applied, domain-specific contexts. Alternatively, if similar results to study 2 were observed, this would suggest that the findings were due to methodological factors associated with the experimental paradigm. To explore this, a final study was conducted (n=80) which examined the effects of the mood incubation procedure in a more domain-general creativity setting. The same incubation paradigm was employed as in study 2 with the exception that participants were from a non design background (mainly psychology students) and the creativity task was Finke's (1996) mental synthesis task. The results suggested that fluency was marginally affected by incubation, in that while both the control group and the neutral incubation group showed a decline in the number of ideas generated in the second phase of the synthesis task, this decline was less pronounced for the incubation group. However, in most key respects the results were similar to study 2: there was no evidence that incubation facilitated novelty, nor was there any evidence to suggest that positive moods were particularly effective during incubation. Finally, as in study 2, a serial order effect was observed with all groups showing higher novelty in the latter half of the task. Overall, the thesis found no evidence to suggest that incubation facilitates subsequent ideation, nor was there any evidence for the notion that positive moods are particularly effective in enhancing ideation. The fact that this was the case for design ideation and domain-general creativity suggests that the findings may be due to specific methodological factors, rather than an incompatibility between incubation and design ideation, as was initially proposed. Regarding the absence of general incubation effects, this could be due to the nature of the incubation task, as well as participants' lack of awareness of a task return following incubation. The lack of positive mood-related effects could be attributable to limitations with the mood induction procedure, or the influence of sketching during ideation. The potential involvement of these factors and how they may have influenced the results is discussed in the relevant chapters. Suggestions are also made for how future research can further explore their role in incubation. Overall, the research highlights that incubation effects can be elusive, and that taking a break is by no means guaranteed to facilitate performance. Finally, the results do provide clear support for the notion that creativity improves with time (serial order effect). Future research should explore the cognitive mechanisms that underlie the serial order effect in a design context, and whether certain factors (e.g. expertise, fluid intelligence) moderate it.