We summarize several decades of work in finding values for the percolation threshold p c for site percolation on the square lattice, the universal correction-to-scaling exponent Ω, and the susceptibility amplitude ratio C + /C − , in two dimensions. Recent studies have yielded the precise values p c = 0.59274602 (4), Ω = 72/91 ≈ 0.791, and C + /C − = 161.5(2.0), resolving long-standing controversies about the last two quantities and verifying the widely used value p c = 0.592746 for the first.