This article discusses the importance of vocabulary in second language acquisition (SLA), presents an overview of major strands of research on vocabulary acquisition, and discusses five principles for effective second language (L2) vocabulary instruction based on research findings on lexical input processing. These principles emphasize presentation of new words as input, allocation of limited processing resources during vocabulary acquisition, distinct components of vocabulary knowledge (e.g., form, meaning, mapping), and appropriate types of instruction for different stages of development. Empirical support is included.