It has looked at the route, the rate, and the end state of SLA, and the ways in which it is affected by external factors such as instruction, interaction, and motivation. Particular areas of interest have included the degree of transfer from the first language/s, the degree of systematicity in learners’ language, variation between learners or within one learner, and―most of all perhaps―why the process of acquiring a second language, as opposed to acquiring a first language, is so often regarded as ‘incomplete’.