Abstract:   Comets are known to often provide spectacular appearances to the ground based observers, with visible tails extending over millions of kilometers, emanating from nuclei of few to 10s of kilometers. The current evidence suggests that the release of dust particles is driven by sublimating (water) ice as the comet approaches the sun. What do we know about their physical nature and activity? Are the nuclei as diverse as their tails? This talk will provide an overview of the current knowledge on the processes responsible for activity, focusing on the ESA Rosetta's results from the rendezvous with the 67P/Churyumov-Garasimenko.