Abstract：Dust grains and ices can be found in a wide range of small Solar System bodies, such as comets and some type of asteroids. Although the population of small icy body is diverse, fundamental similarities con be found among the various classes of objects. Due to small sizes these bodies preserved their ices and primordial porous structure. Comets probably have conserved the most pristine materials that exist today. Therefore, comets are prime targets of space missions. Recently we got a lot of new exiting results from the Rosetta mission to comet Churumov-Gerasimenko/67P, which completely changed our knowledge about these small bodies. Certainly, we found a new amazing world. At the same time， this mission opened a long list of new questions requiring new ideas and new models. The review of the unresolved issues related to cometary activity will be presented in the talk.