The chemical changes induced by radiation, via ionizations and excitations, manifest at the cellular level as effects on cell cycling, a prime focus of this portion of the tutorial.   Effects on in vitro cell colonies are also reviewed in this section.  Organ and organism effects are not covered in this section; instead, these are grouped either by the nature of the risk (i.e. deterministic versus stochastic) associated with each pathology  or with a specific scenario of radiation exposure (i.e. in utero exposure).