Technological change is argued to be taking place along ordered and selective patterns, shaped jointly by technological and scientific principles, and economic and other societal factors. Historical, descriptive analysis is often used to analyze these "trajectories". Recently, quantitative methods have been proposed to map these trajectories. It is argued that such methods have, so far, not been able to illuminate the engineering side of technological trajectories. In order to fill this gap, a methodology proposed by Hummon and Doreian (1989) is used and extended to undertake a citation analysis of patents in the field of fuel cells.