What is moss?

TRACE observations of active regions show a peculiar extreme ultraviolet (E UV) emission over certain plage areas. Termed `moss' for its spongy, low-ly ing, appearance, observations and modeling imply that the phenomenon is cau sed by thermal conduction from 3-5 MK coronal loops overlying the plage: mo ss is the upper transition region emission of hot coronal loops. The spongy appearance is due to the presence of chromospheric jets or `spicules' inte rspersed with the EUV emission elements. High cadence TRACE observations sh ow that the moss EUV elements interact with the chromospheric jets on 10 s time scales. The location of EUV emission in the moss does not correlate we ll to the locations of underlying magnetic elements in the chromosphere and photosphere, implying a complex magnetic topology for coronal loop footpoi nt regions. We summarize here the key observations leading to these conclus ions and discuss new implications for understanding the structuring of the outer solar atmosphere.