This paper develops techniques to locally control curvature and continuity in particle-based surface models. Such models are a generalization of traditional spline surfaces built out of triangular patches. Traditional splines require the topology of the triangular mesh to be specified ahead of time. In contrast, particle-based surface models compute the topology dynamically as a function of the relative node positions, and can add or delete nodes as required. Such models are particularly important in computer vision and other inverse problems, where the topology of the surface being reconstructed is usually not known a priori. We develop techniques for both locally controlling the curvature of the surface (through additional state at each node), and for adapting the triangulation to surface curvature (by concentrating more particles in areas of high curvature). We show how the same ideas can also be applied to 3-D curves, which results in a flexible version of traditional dynamic contours (snakes).