Percutaneous nephrostomy is a mini-invasive technique that creates an external outlet from the renal excretory tract through a catheter inserted through the flank. Indications for this procedure are of both diagnostic and therapeutic type. The nephrostomic catheter is generally positioned under ultrasound guidance, which has the advantages of showing the localization of the renal cavities and the depth of the kidney, and can be used during pregnancy and in subjects with allergy to contrast medium or with reduced renal function. When possible, it is best to associate US with fluoroscopic guidance, as this association has been found to guarantee a success rate exceeding 98%. Within the kidney, the zone known as Broedel's avascular plane, where the terminal branches of the posterior and anterior arterial systems meet, is the safest place to pass the nephrostomic catheter through, as there is little vascularization in this zone. Access is generally posterior and at the level of the inferior calyx, by means of Seldinger's, the one step or a mixed access technique involving a catheter sheathing a metal cannula. Seldinger's access technique is most commonly used, entailing explorative puncture of the renal cavity with a 22 G needle. We prefer to puncture the kidney direct, under US guidance and using an 18 G needle: a metal wire is passed through the needle and then after withdrawing the needle, the fascia dilators are inserted, of scaled widths increasing up to 2 G wider than the nephrostomic catheter. Care must be taken not to go beyond the curve of the guide wire to avoid trauma to the renal parenchyma. The most critical moment is when the catheter is inserted along the wire at the point of passage through the peri-renal fat: the catheter tends to deflect the point of the guide wire away from the desired direction as the peri-renal fat offers less resistance. The metal wire may be of variable rigidity and length, hydrophilic. The catheter may also be made of various materials: the ideal catheter should slide easily, be flexible, soft and resist encrustation. Each material has its own characteristics but polyurethane is the most ductile and is generally used for nephrostomic catheters. The catheter can also feature different types of point and width and may be autostatic like the Foley catheter or else a cope loop.