CURRICULUM TOPIC: INTERVENTIONAL CARDIOLOGY Percutaneous balloon dilatation, first described by Andreas Gruentzig in 1979, was initially performed without the use of guidewires.1 The prototype balloon catheter was developed as a double lumen catheter (one lumen for pressure monitoring or distal perfusion, the other lumen for balloon infla- tion/deflation) with a short fixed and atraumatic guidewire at the tip. Indeed, initially the technique involved advancing a rather rigid balloon catheter freely without much torque control into a coronary artery. Bends, tortuosities, angulations, bifurcations, and eccentric lesions could hardly, if at all, be nego- tiated, resulting in a rather frustrating low proced- ural success rate whenever the initial limited indications ( proximal, short, concentric, non- calcified) were negated.2 Luck was almost as important as expertise, not only for the operator, but also for the patient. It is to the merit of Simpson who, in 1982, introduced the novelty of advancing the balloon catheter over a removable guidewire, which had first been advanced in the target vessel.3 This major technical improvement resulted overnight in a notable increase in the pro- cedural success rate. Guidewires have since evolved into very sophisticated devices. Although they all may look alike from the outside, wires are widely different in their materials, internal structure and design, hence, their wide diversity in function.4 Wiring is of course only one, but not the least, of several steps in coronary intervention, yet the atten- tion given to wire selection is often superficial. This article is aimed at understanding the way wires are constructed, how this influences their specifications, and how to select them for a given purpose. We propose to cover first the technical aspects of guidewires, followed by a rational approach for wire selection.