Macroscopic: V = IR. Resistance R = rhoL/A, where rho is resistivity (ohm-m), L is length, A is cross-section. Microscopic: J = sigmaE, where sigma = 1/rho = $ne^2$*tau/m is conductivity. Ohm's law is empirical — it applies to ohmic materials (metals at constant temperature) where V-I is linear. Non-ohmic materials (diodes, thermistors) don't follow V = IR. R has SI unit ohm (omega) = $\frac{V}{A}$, dimensions [M $L^2$ T^(-3) A^(-2)]. Conductance G = 1/R, unit siemens (S). Resistivity is an intrinsic material property (independent of geometry); resistance depends on both material and geometry.