This paper supports the view that language instruction should be evidence and principle-based. Key facts about second language acquisition will be presented highlighting the main implications for language instruction. A principled and evidence-based approach to language instruction is one which takes into consideration the following: (i) a clear understanding of what language is; (ii) a clear understanding of what communication means; (iii) an effective approach to focus on form is one which ties together form and meaning and moves from input to output practice; (iv) input is the key ingredient in language acquisition and L2 learners needs to be exposed to comprehensible and meaningful language input during instruction; (v) interactive tasks might play a facilitative role in the acquisition of a second language.