Two key research areas must advance before sketch-based interfaces can transition from a collection of individually provocative research prototypes into a computing platform of choice. First, fundamental techniques for segmenting, recognizing, and parsing collections of ink strokes must be generalized and made more robust with better user models. Second, developers must go beyond redesign and actually reinvent their applications to leverage pen input's intrinsic capacity for rapid, direct, modeless 2D expression. Solving either of these problems independently is no guarantee of success. Guest editors Takeo Igarashi of the University of Tokyo and Bob Zeleznik of Brown University introduce this special issue, which reflects on these two research themes.