Blazar is a subclass of radio-loud AGNs which shows flux and spectral variability in the complete electromagnetic (EM) spectrum and variable polarization from radio to optical bands. Blazar emission in the complete EM spectrum is pre-dominantly non-thermal. In the recent age of multi-wavelength astronomy, blazars are among one of the most favorable targets to do simultaneous multi-wavelength observations for extended periods of time to understand the emission mechanism responsible in this class of AGNs. In the present talk I will present results based on multi-wavelength flux and spectral variability, and quasi-periodic oscillations (QPOs) obtained by us in various blazars using various ground and space based telescopes.