Interaction Grammars are a grammatical formalism based on the notion of polarities. Polarities express the resource sensitivity of natural languages by modelling the distinction between saturated and unsaturated syntactic structures. A sentence's syntactic composition is represented as an ``electrostatic'' process guided by the neutralization of polarities. At the heart of the formalism, there is also the notion of underspecification: syntactic structures are not completely specified trees but tree descriptions. Then, parsing appears as a process of building tree description models. Semantics is represented at a level independent of the syntactic level, but based on the same notions of polarity and description with a difference: descriptions are not tree descriptions but directed acyclic graph (DAG) descriptions. The interface between the two levels is realized in a flexible way by a function that links every syntactic node to at most one semantic node.