Tasks lie at the centre of learning and teaching mathematics. Tasks bring mathematical activity into lessons, by which I mean mental activity, not just physical activity. Tasks have to meet the needs of specific pupils, so some writers encourage teachers to design their own tasks. As a former primary school teacher I know that time demands make this a high expectation, and having devoted my time teaching in higher education exclusively to mathematics education, I’ve also come to appreciate just how difficult task design is. Besides, there is such a plethora of tasks now available on the Internet I am not sure any more need creating. But not all tasks are created equal. I think it is a more reasonable expectation that teachers need to make good choices between tasks and a primary aim of this book is to support that decision making.