The most striking fact about second-language learning, especially as compared with first-language learning, is the variability in outcomes. Many individual and group variables have been examined in attempts to explain success or failure in second-language acquisition. This section reviews the literature on various individual differences in second-language acquisition. In looking at this literature, it is important to appreciate that the definition of the outcome of the second-language acquisition process has itself been variable, as discussed in the previous section (see also Chapter 5 on student assessment).