  We give a short proof of the fundamental result that the critical probability
for bond percolation in the planar square lattice is equal to 1/2. The lower
bound was proved by Harris, who showed in 1960 that percolation does not occur
at $p=1/2$. The other, more difficult, bound was proved by Kesten, who showed
in 1980 that percolation does occur for any $p>1/2$.
