Metasurfaces, ultrathin metamaterials composed by array of planar “meta-atoms” with pre-designed electromagnetic (EM) properties, have attracted extensive attention recently.  However, so far moat realized metasurfaces are passive and their working efficiencies are not high.  In this talk, I will summarize our recent efforts in this field, emphasizing mainly on the tunability and efficiency issues of them.  In particular, I will show how to make tunable metasurfaces to actively control EM waves, in both THz and GHz frequency domains, and how to make ultrathin pancharatam-berry metasurfaces with nearly 100% efficiencies, in both reflection and transmission geometries.  Finally, I will also introduce our recent efforts on utilizing metasurfaces to realize certain applications, such as broad-band diffusive scatterings, SPP couplers, and so on.