Topological order (Wen 1989) is a type of quantum order that cannot be described by local order parameters or symmetry breaking. The toric code (Kitaev 1997) places qubits on lattice edges with stabilizers A_v = ∏ σ^x (vertex stars) and B_p = ∏ σ^z (plaquettes). Excitations come in pairs of anyons — e-anyons (vertex violations) and m-anyons (plaquette violations) — that are mutual semions (π phase under exchange). The ground state degeneracy is 4-fold on a torus and is topologically protected against local perturbations, making it a candidate for fault-tolerant quantum memory.