Rate = k (constant, independent of concentration). Integrated rate law: [A] = [A]_0 - kt. Plot: [A] vs t is a straight line with slope = -k, intercept = [A]0. Half-life: $t_1$/2 = [A]$\frac{0}{2k}$ — proportional to initial concentration. The reaction proceeds at constant rate until reactant is completely consumed, then rate drops to zero abruptly. Examples: decomposition of NH3 on hot Pt surface, decomposition of HI on gold surface, photochemical reactions (rate depends on light intensity, not concentration). Enzyme-catalysed reactions at high substrate concentration (enzyme saturated) also show zero-order behaviour. Zero order is relatively rare compared to first and second order.