Implicit surfaces offer many advantages for sketch- based modeling systems, such as blending, CSG, and a procedural object hierarchy. Free-form deformation (FFD) is also extremely useful in this context, however existing FFD approaches do not support implicit surface representations, and FFD lattice manipulation is time-consuming compared to sketch- based techniques. In this thesis, an FFD technique suitable for implicit surface representations is described. To enhance real-time feedback, the problem is split into an approximate formulation used during interactive deformation, and a more robust variational technique which preserves desirable scalar field properties. As an interface to manipulate the deformation, a sketch-based volumetric peeling interface is introduced. The user's task is to draw a curve on the surface, and pull or push the surface to the desirable position via the curve. Subsequently, the deformation is automatically defined. This technique has been implemented in a prototype implicit FFD system called Taco. Results created in Taco show that a desirable deformation can be easily achieved while preserving implicit properties.