We discuss the role and importance of sketching during conceptual design ideation and position it as instrumental in understanding what it means to design. To do this we first define design as a generative, transformative act. We then situate sketching as an effective means through which the transformative requirement of design is achieved as reason for its prolific use in design. Following this, in order to ground our theoretical discussion, two examples of sketch work are presented and discussed. The two examples provide an illustration of how sketching is used to both resolve the indeterminacy of the conceptual design situation, and establish a means by which the designer may navigate a design solution. Finally, we reflect upon the potential of a focus upon sketch representation to contribute to developing an understanding of what it means to design and implications for efforts towards building a philosophy of design.