Dimensionless fracture conductivity is a key design parameter in well stimulation that compares the capacity of the fracture to transmit fluids down the fracture and into the wellbore with the ability of the formation to deliver fluid into the fracture. Its use in fracture design dates back to the earliest days of hydraulic fracturing in the 1950's. Despite the advances in numerical simulation and, perhaps, because of its relative simplicity in presenting such an important concept, it continues to be used as a principal design parameter some 40 years later However, as with any design parameter, its value is only as good as the data used in its determination. Both recent testing reported in the literature and other studies from as early as the 1970's show that the source of fracture-conductivity data used by most engineers seriously overestimates the effective permeability of the fracture by more than an order of magnitude. This results in many of today's hydraulic-fracture treatments being capacity constrained and significantly underperforming compared with the potential deliverability from the reservoir. Understanding the test parameters under which the input data are measured enables today's engineers to calculate dimensionless fracture conductivity more accurately and improve the productivity from their wells.