I will discuss state of the art population synthesis models that improve on the following aspects with respect to most available models. (a) Treatment of the UV spectral range, including new evolutionary tracks and updated treatment of the ionizing radiation emitted by the stellar population. (b) Improved treatment of TP-AGB stars that dominate the NIR spectral range.  (c) Definition of new diagnostics to characterize the combined stellar and nebular emission and the cold ISM in galaxies. (d) The role of interacting binary stars on the integrated spectra of stellar populations will be discussed and compared with the effects produced by stochastic fluctuations in low mass populations. Single star models are available in a wide range of metallicity, from Z = 0 to Z = 0.06, and are ready to use to interpret the spectra of galaxies of any age at low and high redshift. Applications will be discussed.