Joint ATNF/AAO Colloquium. Wednesday, 20th September - 3:30pm ATNF Lecture Theatre Theoretical Modelling of HII regions and Warm Infrared Galaxies Lisa Kewley RSAA, Australian National University I will present photoionisation and shock models of the extragalactic HII region abundance sequence and warm infrared galaxies. These models use the stellar population synthesis models PEGASE and STARBURST99 to produce the EUV ionizing radiation field used by our photoionization and shock model MAPPINGs. I will discuss the usefulness of optical diagnostic diagrams in determining the metallicity and ionization paramater of HII regions and starburst galaxies, and explore the effect of using different stellar tracks and stellar atmosphere models. Finally, these models are used to derive a new technique to classify objects into starburst, AGN and LINER types and to determine objects which are possibly composite in nature. Many galaxies previously classified as LINERs appear to lie on a mixing line between starburst and AGN type galaxies and are likely to be composite in nature. Our theory-based classification scheme is compared with the previous semi-empirical scheme of Veilleux and Osterbrock (1987) scheme and we find that our classification method results in 6% ambiguity in classifications between the different diagnostic diagrams compared with 16% ambiguity using the traditional Veilleux and Osterbrock (1987) method.