The electronic absorption and emission spectra of acridine have been studied by means of a multiconfigurational second-order perturbation method (CASSCF/CASPT2) and its multistate extension (MS-CASPT2). The low-lying valence singlet and triplet pi - pi* and n - pi* excited states have been computed. The location of the lowest Rydberg state (3s) has been also estimated. By optimization of the geometries of the ground and low-lying excited states and the calculation of transition energies and properties, the obtained results lead to a complete analysis and assignment of the available experimental singlet-singlet and triplet-triplet absorption spectra and to the description of the basic features of the fluorescence and phosphorescence processes of acridine. The photophysics of acridine and its protonated form are analyzed and the effects of solvation are discussed. The present findings support the model of a state reversal on the lowest singlet excited state upon increasing the solvent polarity.