Abstract. In r-neighbour bootstrap percolation on a graph G, a set of initially infected vertices A ⊂ V (G) is chosen independently at random, with density p, and new vertices are subsequently infected if they have at least r infected neighbours. The set A is said to percolate if eventually all vertices are infected. Our aim is to understand this process on the grid, [n] d , for arbitrary functions n = n(t), d = d(t) and r = r(t), as t → ∞. The main question is to determine the critical probability p c ([n] d , r) at which percolation becomes likely, and to give bounds on the size of the critical window. In this paper we study this problem when r = 2, for all functions n and d satisfying d log n. The bootstrap process has been extensively studied on [n] d when d is a fixed constant and 2 r d, and in these cases p c ([n] d , r) has recently been determined up to a factor of 1 + o(1) as n → ∞. At the other end of the scale, Balogh and Bollobás determined p c ([2] d , 2) up to a constant factor, and Balogh, Bollobás and Morris determined2+ε , and gave much sharper bounds for the hypercube. Here we prove the following result: let λ be the smallest positive root of the equation