We present an empirical study that illustrates the potential of dual eye-tracking to detect successful understanding and social processes during pair programming. The gaze of forty pairs of programmers was recorded during a program understanding task. An analysis of the gaze transitions between structural elements of the code, declarations of identiﬁers and expressions shows that pairs with better understanding do less systematic execution of the code and more “tracing” of the data ﬂow by alternating between identiﬁers and expressions. Interaction consists of moments where partners’ attention converges on the same same part of the code and moments where it diverges. Moments of convergence are accompanied by more systematic execution of the code and less transitions among identiﬁers and expressions.