Absorption line-strengths have allowed us to carry out a comprehensive stellar populations analysis, providing very strong constraints for discriminating and constraining current galaxy formation scenarios. I describe the main features of this approach, pinpointing the most popular indices and diagnostic diagrams, and the main problems affecting these studies such as the age-metallicity degeneracy. A special emphasis is given to ongoing efforts aiming at defining more accurate diagnostics, which allow us to fully exploit the flow of high quality data from current generation of large telescopes, including GTC.