In the solar spectrum the Mg II h & k are peculiar lines. They stand out by being very strong and by their peculiar shape, defying normal classification. Their position in the UV makes them all but non observable from the ground, remaining an exotic oddity for decades, sporadically observed by balloon or rocket observatories. The launch of the IRIS mission has changed this status quo. Frequent, high-resolution observations from the IRIS spectrograph have spearheaded their move from a spectroscopic backwater into a prime diagnostic of the solar chromosphere. But what do their spectra tell us, and how do we interpret them? I will show how 3D MHD simulations of the solar atmosphere have helped to identify key diagnostic properties of the h & k and subordinate lines, and how the lines can complement existing observations. The lines provide a fascinating view of the solar chromosphere; I will show some examples from IRIS observations, in particular how they helped to piece together the evolution of solar spicules.