A multiwavelength vision of star forming regions: What does lambda_1 know about lambda_2?

In this contribution I show the physical limitations to the application of standard synthesis models from 0 to 10 Gyr. I also present the multiwavelength spectrum (from gamma-rays to radio) of young star forming regions based on evolutionary population synthesis models that takes into account the stochasticity of the star formation proces. I show how the correlation of different observables can allow to establish a better understanding of the star formation processes. Finally I compare the spectral energy distribution of AGNs and star forming regions and I show the possible connection of Star Forming Galaxies with Seyfert 2 AGNs.