This paper will review the main positions around the role of grammar instruction which have emerged from studies conducted to investigate the effects of formal instruction in second language acquisition (Van Patten and Benati, 2010). In the last twenty years researchers have examined the issue of how learners interact with input asking questions such as “why do they skip over some things in the input?” and “what makes some features harder to process than others?” (Van Patten, 2005). Such questions drove researchers to examine the effects not of instruction more generally but of particular kinds of instructional interventions such as input enhancement, input flood and in particular processing instruction (Wong, 2005). The paper will track the impact processing instruction has made since its conception (Lee and Benati, 2009). It will explain processing instruction, both its main theoretical underpinnings as well as the guidelines for developing structured input practices. It will also provide an overview of the empirical research conducted to date, on processing instruction and it will reflect on the new research trends on measuring the relative effects of this instructional approach to grammar instruction (Benati and Lee, 2008, 2010).