Dual snake models are powerful techniques for boundary extraction and segmentation of 2D medical images. In these methods one contour contracts from outside the target and another one expands from inside as a balanced technique with the ability to reject local minima. Such approach was originally proposed in the context of parametric snakes. Recently, two implicit formulation for dual snakes were presented: our proposal, called the Dual-Level-Set, and the Dual-Front approach. In this paper we review these methods and offer some comparisons. We survey applications for shape recovery in 2D cell and human brain MRI images.