Due to the complexity of their environment, prominences properties are still a matter of controversy. Prominences cool and dense plasma is suspended in the hot corona by a magnetic structure poorly known. Their thermal insulation from the corona results in a thin geometrical interface called prominence-corona-transition-region (PCTR). Here we will review the main properties of such a region as derived primarily from observations. We will introduce the thermal structure properties, describe the fine structure together with the Doppler-shift and width properties of lines of the emitting plasma. We will introduce the proposed interpretations of such observations and the limits of our knowledge imposed by the present instrumentation. We will conclude with a perspective for the future observations of the PCTR.
