Stressing the importance of theory might be considered a sign of indifference to educational data. But of course the contrary is the real case. Careful attention to achieving the most probable theory is the best way to take account of the greatest possible amount of relevant data. It’s the best way of not being diverted from sound educational policy by some fresh bulletin from the schools that may or may not truly show what it claims to show. Suppose somebody comes up with a claim for a program that, according to research, can bring a child from low language comprehension to proficient language comprehension in one school year. This is not, perhaps, an absolute impossibility, but on strong theoretical grounds having to do with the gradual nature of knowledge and vocabulary acquisition, we need to be especially wary of claims to quick fixes in reading proficiency. There is a lot of evidence that although language development can be accelerated, it can never be really fast. We know this because we are beginning to have a deeper understanding of the way vocabulary and its accompanying knowledge is built up. This theoretical understanding can enable us to speed up progress in near-optimal fashion even as it repudiates the notion that Seabiscuit-style progress is possible in reading comprehension.