<qa>  What is the main challenge to researchers in this domain?- Background : Despite the availability of both pharmacological and psychosocial treatments, most affected individuals will experience addiction as a chronic illness with severe consequences for health outcome. A fundamental challenge to researchers in this domain is therefore to understand the mechanisms leading to relapse. Among the key vulnerability factors associated with addiction chronicity, increasing attention has focused on deficits in executive functions and associated traits impulsivity, risk taking that are likely to explain why many individuals with addiction are unable to avoid high-risk contexts or to manage the experience of craving. Research concerning this vulnerability to relapse has been hindered by the manner in which cognitive deficits are traditionally assessed in clinical research. Most commonly, they are thought to represent stable risk factors that show little or no variation within a given individual. However, executive functioning may fluctuate rapidly over time as does attention, memory, or other key cognitive functions, and therefore comparisons of overall or mean cognitive deficits between healthy and substance-dependent individuals may show no significant differences despite powerful effects of this vulnerability. Therefore, the absence of measurement or characterization of this important relapse mechanism prevents any further research concerning its underlying pathophysiology. 