This paper provides a critical review of Processing Instruction (PI). This type of instructional option was specifically designed to help second/foreign language (L2) learners grasp meaning manifested in the use of particular grammatical forms in a target language effectively through the provision of input. In this way, PI attempts to help learners develop grammatical competence in a target language in ways that foster their comprehension and production skills. The paper starts with outlining main characteristics of PI which distinguish it from other types of L2 grammar instruction. Then, a large body of research attempting to investigate the relative efficacy of PI is scrutinized. The paper concludes with a number of important issues that future studies on PI need to address.