Preliminary design ideas, mostly not manifest, are generated in the designers’ minds at the very early stages of their encounter with the design problems. This paper reports a study of the sources from which these ideas are originated, and the corresponding role each source plays in the process of designing. The study was undertaken through the analysis of semi-structured interviews with eight Iranian professional architects about the ideation process of at least one of their design projects. To complement the data gathered through interviews, each interview was followed by a discussion session within a group of graduate architecture students, reflecting on the narratives as well as the documents presented by the participating architects. The interviews as well as the discussion sessions were administered within the framework of a graduate course unit entitled ‘Ideation and the Design Process’, taking a time span of two academic semesters. Graduate students, set in groups of two to five members, contributed to conducting the interviews and further preparing the corresponding written documents. Each group was briefed to interview one of the participating architects, assuring to cover seven key questions of a pre-designed interview pro forma; the questions aiming to uncover the participants’ trains of thought leading to the generation of the central design ideas, and further design developments. In a number of cases, a complementary interview was conducted by the authors to corroborate the written documents prepared by students. A content analysis of the architects’ statements, transcribed from either the interviews or the discussion sessions, was used to identify the role of different sources of design ideas in each design situation. An axial coding of the contributing factors to design ideation, as declared by the architects, verified the central role of the design situation in due course. Three main contributing factors to any design situation were further categorized into ‘the factors brought in by the designer’ and ‘the factors brought in by the design problem’, the latter covering both ‘the site attributes’, and ‘the design brief’ of any design situation. Frequently pronounced by the participating architects, the above mentioned factors can be reckoned as the main sources of design ideation. The findings of the study suggest that the ‘design problems’ were, in sixty percent of the cases, the main source for the design ideation. Design ideas, though, seem to be generated mostly through contemplating the problem situation, i.e. the content and the context of the design task. It was also indicated by the findings that a rating of forty percent of design ideas was evidently brought to the situation by the designers, prior to contemplating the problem variables. In such cases, the designers proved to have drawn on a personal set of cognitive and affective constructs, implying their knowledge and attitude, to come up with the central design ideas. Having acknowledged the changing degree to which any source contributes to idea generation, the paper concludes with a three-fold model which might be used to demonstrate the corresponding roles the sources play in any design situation.