This chapter provides an introduction to the second set of key descriptive facts about second language acquisition which is related to learner variation. Just because all learners follow the same stages of acquisition does not imply that they all display exactly the same linguistic features. Instead, learners develop different styles at every level of acquisition. This chapter gives an overview of how these learner styles are linked to stages of acquisition and which linguistic mechanisms permit them to emerge. Again, the study questions give the reader an opportunity to apply descriptive statements about ESL variation to the analysis of authentic learner language.