Reaction times (RTs) have been of interest to empirical psychology for as long as the discipline has existed. However, most studies summarise RTs using measures of central tendency, ignoring high levels of intra-subject variability (ISV). ISV has recently become an important topic in neuroscience and differential psychology in its own right, showing strong associations with intelligence, cognitive ageing, and several psychiatric and neurological disorders. The present thesis describes a programme of research into the measurement of individual differences in ISV, using a mixture of psychometric and psychophysiological techniques. Study One explores measurement issues, comparing various metrics of ISV in terms of their reliability and statistical redundancy. Study Two compares the single-trial event-related potentials of participants with high and low levels ofISV on a working memory oddball task. Study Three looks at the cross-task and cross-modal structure ofISV, using a latent variable approach. Study Four integrates approaches from the previous three chapters, deriving supra-task latent variables for ISV and several single-trial P3b parameters, and exploring the latent variable correlations between these constructs. These studies suggest that a latent variable framework may be a promising framework for studying ISV, not just in behavioural studies but in also psychophysiological research. Possible implications for future research are discussed, based on the empirical and theoretical portions of the thesis.