  We discuss Manin's conjecture concerning the distribution of rational points
of bounded height on Del Pezzo surfaces, and its refinement by Peyre, and
explain applications of universal torsors to counting problems. To illustrate
the method, we provide a proof of Manin's conjecture for the unique split
singular quartic Del Pezzo surface with a singularity of type D4. (Lectures at
the summer school "Equidistribution in number theory", Montreal, August 2005)
