For metals: R = $R_0$(1 + alpha*$delta_T$), where alpha ~ 3-5 x 10^(-3) /K. Resistance increases with T because lattice vibrations increase, reducing relaxation time tau. For semiconductors and insulators: resistance decreases with T because more carriers are thermally excited across the band gap. Superconductors: R drops to exactly zero below the critical temperature $T_c$. Carbon (graphite) has negative temperature coefficient (resistance decreases with T). Alloys like manganin and constantan have very small alpha — used in standard resistors. At absolute zero, pure metals have very low but non-zero resistance (due to impurities); superconductors have exactly zero.