Particle number fluctuations and correlations in nucleus-nucleus collisions at SPS and RHIC energies are studied within the statistical hadron-resonance gas model in different statistical ensembles and in the Hadron-String-Dynamics (HSD) transport ap
proach. Event-by-event fluctuations of the proton to pion and kaon to proton number ratios are calculated in the HSD model for the samples of most central collision events and compared with the available experimental data. The role of the experimental acceptance and centrality selection is discussed.
