Two new criteria, that involve the microscopic dynamics of the system, are proposed for the identification of clusters in continuum systems. The first one considers a residence time in the definition of the bond between pairs of particles, whereas the second one uses a life time in the definition of an aggregate. Because of the qualitative features of the clusters yielded by the criteria we call them chemical and physical clusters, respectively. Molecular dynamics results for a Lennard-Jones system and general connectivity theories are presented.