Melting experiments require rapid data acquisition due to instabilities of the molten sample and optical drifting due to the high required laser power. In this work, the melting curve of zirconium has been determined for the first time up to 80 GPa and 4000 K using in-situ fast x-ray diffraction (XRD) in a laser-heated diamond anvil cell (LH-DAC). The main method used for melt detection was the direct observance of liquid diffuse scattering (LDS) in the XRD patterns and it has been proven to be a reliable melting diagnostic. The effectiveness of other melting criteria such as the appearance of temperature plateaus with increasing laser power is also discussed. PACS numbers: 92.60.hv, 61.50.Ks, 74.62.Fj, 81.30.Bx Zirconium (Zr) and its alloys have a very wide range of applications, from the chemical processing (as corrosion resistant materials) to the semiconductor industry 1 . Moreover, its good strength and ductility at high temperatures and the low thermal neutron cross-section absorption make it an ideal material for use as cladding at nuclear reactors 2 . Alloys of Zr with Cu, Al, Ti and Ni have been demonstrated to exhibit extraordinary glass forming ability 3 , while metallic glass formation in singleelement zirconium has also been discovered, with a wide stability in high pressure and temperature conditions 4 .Zirconium is a d-orbital transition metal with a rich and interesting phase diagram. At ambient conditions it crystallizes to an hcp structure (α-phase), while at temperatures higher than 1136 K it transforms to a bcc (β-) phase. By increasing pressure at ambient temperature it transforms to another hexagonal, but not close-packed, called the ω-phase and then back to β-phase around 35 GPa 5-7 . Similar transitions also occur in other group IV transition metals, such as Ti and Hf, and it seems that the electronic transfer between the broad sp band and the much narrower d band is the driving force behind those structural transitions.The high melting point (2128 K) of Zr often classifies it as a refractory metal. Although there are some works in the high temperature behavior of zirconium at high pressures 8-11 , its melting curve has not yet been investigated and this absence of experimental data has strongly motivated this study. On the other hand, the high pressure melting of transition metals has always been a subject of intense debate, because of the large uncertainties in the temperature measurements and the criteria used to identify the melting, so that different approaches can yield very different results. In most cases shock wave (SW) experiments and molecular dynamics (MD) calculations provide dramatically steeper curves than those obtained with the laser speckle method in a LH-DAC, where the melting is visually detected by observing the movements on the sample surface during heating. Tantalum is a good example of such a controversy, with melt-ing temperatures that differ thousands of K at 100 GPa by applying different experimental techniques [12][13][14][15] . Another more recent example is...