The Engine Combustion Network (ECN) is a worldwide group of institutions using combustion vessels and/or performing computational fluid dynamics (CFD) simulation, whose aim is to advance the state of spray and combustion knowledge at engine-relevant conditions. A key activity is the use of spray chamber facilities that operate at high-temperature, high-pressure conditions typical of diesel combustion, which are operated at specific target conditions in order to leverage research capabilities and advanced diagnostics of all ECN participants. The first target condition, called "Spray A," has been defined with detailed ambient (900 K, 60 bar, 22.8 kg/m(3), 15% oxygen) and injector (common rail, 1500 bar, KS1.5/86 nozzle, 0.090-mm orifice diameter, n-dodecane, 363 K) conditions. Establishing and improving these experimental boundary conditions in unique facilities throughout the world represents a major step forward in the establishment of high-quality, quantitative data sets for engine spray combustion. This paper is a review of the methodology to characterize and control the ambient and fuel-injector boundary conditions (e.g., temperature, pressure, composition) as offered by six different participating institutions of the ECN, each targeting the Spray A conditions and quantifying experimental uncertainty. Constant-pressure flow (CPF) and constant-volume preburn (CVP) chambers with various ambient gas composition are compared for the first time. Experimental diagnostics include the use of fast-response, radiation-corrected thermocouples for spatially resolved gas and fuel-injector temperature, laser-induced phosphorescence for surface temperature, and high-speed transducers for pressure. With guidance about the uncertainty and variation that exists between facilities, simplified models are then employed to understand how these boundary condition variations may affect aspects of spray combustion. Ambient gas and fuel temperature effects on liquid- and vapor-phase penetration are examined with established one-dimensional models. Chemical kinetics modeling in single- or multi-zone reactors is used to predict the influence of different preburn environments on the major and minor species present in the ambient gas at the start of injection, and their subsequent effect on spray ignition. This review article provides recognition of the challenge in creating well-controlled high-temperature, high-pressure environments, and identifies which boundary condition variations are expected to have the highest impact on spray combustion.