We show how to model geometric patterns on surfaces. We build on the concept of shape grammars to allow the grammars to be guided by a vector or tensor field. Our approach affords greater artistic freedom in design and enables the use of grammars to create patterns on manifold surfaces. We show several application examples in visualization, anisotropic tiling of mosaics, and geometry synthesis on surfaces. In contrast to previous work, we can create patterns that adapt to the underlying surface rather than distorting the geometry with a texture parameterization. Additionally, we are the first to model patterns with a global structure thanks to the ability to derive field-guided shape grammars on surfaces.