||In this talk I will highlight the key observations of solar prominences and filaments that have been obtained with the Interface Region Imaging Spectrograph mission, and some of the results and outstanding questions arising from these observations. Emphasis will be placed on the diagnostic potential of the Mg II h and k lines from the detailed study of their line shapes. With the availability of other strong lines observed by IRIS, and with joint observations obtained during coordinated campaigns, I’ll describe what we have learnt and what puzzles remain. Interpretation of the Mg II h and k line profiles and intensities relies heavily on radiative transfer models, and I will cover the progress made in that area. Finally, some perspectives on future observations will be given.