  This paper reviews the theoretical modelling of starburst galaxies with
particular emphasis on the importance of the gas-phase physics in determining
the form of the continuum SED and the emission line spectrum. Both line and
continuum diagnostics have been used as star formation indicators. Here, we
examine from the point of view of the theory how reliable each of these
indicators may be, and to what physics of the galaxy they are sensitive, with
the aim of decreasing the errors associated with measuring the star formation
history of the universe.
