Temperament refers to early-appearing variation in emotional reactivity. The core dimensions of temperament and optimal method for assessment continue to be sources of considerable discussion. Nevertheless, the moderate stability of most temperamental traits and the strong influence of genetic and unique environmental factors have been well established, along with temperament's association with childhood psychiatric disorders. Both temperamental predisposition toward experiencing negative emotions and low inhibitory control are linked to many psychiatric conditions, while other dimensions, such as levels of extraversion, vary by, and likely even within, disorders. Accumulating research directed at understanding the mechanism of these links between temperament and psychopathology indicate that, at least for most disorders, the two constructs cannot be viewed as simply different points along a shared continuum. The effect of temperament upon psychopathology has been found to be mediated and moderated by a number of both internal and external factors. Additional research is needed to help further define the core dimensions of temperament and the complex mechanisms through which temperamental traits interact with other influences in affecting developmental trajectories.