The role of grammar in reading comprehension remains unresolved despite the relatively large number of studies in this line of research. Drawing on the (dis)advantages of applying grammatical knowledge to the reading process, the present study discusses some of the reasons for differing results reported by previous studies. It is argued that grammatical knowledge can affect comprehension differently under different conditions. Specifically, grammar can be helpful in understanding complex sentences, while being even counterproductive when applied to texts with simpler structures. It is, therefore, important to take into consideration the interaction between grammatical knowledge and the text when examining the role of grammar in reading comprehension. An operational framework of grammatical knowledge with regard to reading comprehension is put forward to illuminate its role in reading comprehension and provide guidelines for teaching practice. The operational framework makes a primary distinction between grammatical knowledge and grammar instruction and further places them on the implicit/explicit continuum. It is concluded that knowing how and when to resort to grammar is as important for students as being competent in grammar. This conclusion highlights the significance of adopting a strategy-based approach to grammar instruction. Teachers should teach both cognitive and metacognitive strategies when teaching grammar for reading.