Engineering Crystal Symmetry and Polar Order in Molecular Host Frameworks Manipulation of organic solid state structure is generally limited to only gross structural features. With partial support from the MRSEC Seed program, host frameworks, constructed by connecting flexible hydrogen-bonded sheets with "banana-shaped" pillars, were found to sustain one-dimensional channels that were occupied by guest molecules during crystallization. The polar host frameworks enforced the alignment of these guests into polar arrays, with properly chosen guests affording inclusion compounds that exhibit second harmonic generation because of this alignment. This protocol exemplified a principle goal of modern organic solid-state chemistry, namely, the precise control of crystal symmetry and structure for the attainment of a specific bulk property. [K.T. Holman, A.M. Pivovar, M.D. Ward, Science 2001, 294, 1907]