Dynamics in the cosmological mass function (or, why does the Press &   Schechter work?)
  The Press & Schechter ``numerical recipe'' is briefly reviewed, together with the recently proposed dynamical mass function theory, in which the mass function is constructed by using the powerful Lagrangian perturbation theory. The dynamical mass function is found in good agreement with the recent N-body simulations of Governato et al. (1998), in the case of an Einstein-de Sitter Universe. The definition of collapse, the relation between mass and smoothing radius, and the definition of structure in 1D Universes are discussed. A detailed comparison of the dynamical mass function to simulations reveals that the orbit-crossed regions in the simulation are correctly reproduced, while the fragmentation of the collapsed medium into structures cannot be done in a univocal way. Finally, we try to answer the question: why the hell does the Press & Schechter work? 
Pierluigi Monaco (Institute of Astronomy, Cambridge, UK)