  A new model that describes adsorption and clustering of particles on a
surface is introduced. A {\it clustering} transition is found which separates
between a phase of weakly correlated particle distributions and a phase of
strongly correlated distributions in which the particles form localized fractal
clusters. The order parameter of the transition is identified and the fractal
nature of both phases is examined. The model is relevant to a large class of
clustering phenomena such as aggregation and growth on surfaces, population
distribution in cities, plant and bacterial colonies as well as gravitational
clustering.
