A spin glass has random exchange couplings J_ij ~ N(0,σ²). Competing ferromagnetic and antiferromagnetic bonds create frustration — not all bonds can be simultaneously satisfied. The Parisi solution (1979) uses replica symmetry breaking: exponentially many metastable states separated by barriers. The order parameter q = (1/N)Σ⟨σ_i⟩² > 0 below T_g.