Following the publication of an article summarising two systematic reviews on whether the teaching of grammar is effective in improving 5-16 year olds' writing (Andrews et al, 2006), the present article reflects on the relationship between knowledge about language and knowledge about grammar. It argues that there is no evidence for the assumption made by policy-makers and researchers in the UK that knowledge about grammar is a useful tool in helping school pupils to write more fluently and accurately. More research is needed to determine which approaches to sentence construction are effective in helping pupils in these respects; and there is an urgent need for curriculum reform on this topic. (Contains 1 footnote.)