Leopar {http://leopar.loria.fr) is a parser for natural languages which is based on the formalism of Interaction Grammars (IG). The parsing principle (called "electrostatic parsing") consists in neutralizing opposite polarities: a positive polarity corresponds to an available linguistic feature and a negative one to an expected feature. The structures used in IG are underspecified syntactic trees decorated with polarities and called Polarized Tree Descriptions.