The emission line spectra of active galactic nuclei fall into three main categories: star formation nuclei (or HII regions), Seyfert nuclei and LINERs (low ionization narrow emission-line regions). A subset of these galactic nuclei has intermediate characteristics between LINERs and HII regions. These so-called transition objects were the subject of numerous studies in the past, but their true nature has remained elusive. It's almost impossible to explain these objects with models that take into account only one ionization mechanism, being it stars, an AGN or shocks. We show how the use of a set of elaborated photoionization models, that account consistently for these parameters (ionization by stars, shocks and/or AGN and dust), constrained by a detailed study of the stellar population properties, can help us understand the nature of these transition objects.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html