Chemostat (Novick & Szilard 1950, Monod 1950): A continuous culture device where fresh medium (substrate concentration S₀) flows in at dilution rate D and culture flows out at the same rate. The Monod growth law μ(S) = μ_max S/(K_s+S) gives: dS/dt = D(S₀−S) − μ(S)X/Y; dX/dt = (μ(S)−D)X. The non-trivial steady state has S* = K_s D/(μ_max−D) (washout if D>μ_max). Competitive exclusion principle: in a chemostat, only the strain with the lowest S* survives competition — the rest are washed out. This is a mechanistic derivation of the exclusion principle from first principles.