This chapter distinguishes studies on the mind from studies investigating the brain. By describing linguistic, psychological, and cognitive neuroscience approaches, some aspects which have been found to be relevant for bilingualism have only been studied in linguistic terms, leaving open whether certain findings are limited to the mind level only or whether there is any correspondence on the brain level. Other aspects, well researched in linguistic or psychological studies, have not yet been taken up in neuroscientific studies, leaving the question of whether certain variables would change the results or explain variance unanswered. We then look at details of learning in the brain and in particular at brain plasticity, a lifelong available characteristic of the brain. The chapter also addresses the notion that for linguists, acquisition and learning are not the same, since context in general and factors such as input quality and quantity must be taken into account. In brain terms, there is only learning. We then discuss different factors influencing language acquisition and learning and reveal that individual differences can be found among these factors to a great extent.