The model-driven architecture approach (MDA) promotes software development as driven by a thorough modeling phase where application code is automatically generated from a platform specific UML model (PSM). The idea is that the PSM is itself derived from a platform independent UML model (PIM). Such code generation and model derivation are examples of model-to-model transformations. This paper presents the Visual Model Transformation (VMT) approach, which includes a transformation language and a tool to support UML model transformations. The transformation language is a visual declarative language that supports the specification, composition and reuse of model transformation rules. These rules make use of the OCL language and a visual notation to indicate the selection, creation, modification and removal of model elements. An abstract denotational semantics based on graph transformation is sketched for the VMT language. We also present the MEDAL tool, which is a prototype build on top of IBM/Rational XDE development environment, and is a first step towards tool support for the VMT approach.