ABSTRACT This paper studies the impact of localization, urbanization, and diversification on regional labor productivity levels and growth. We find substantial effects, accounting for roughly half of the explained variation in the labor productivity differences within the Netherlands in the 1990s. Diversification, urbanization, and localization effects are significant and positive for productivity levels. These levels appear cointegrated. The error correction specification of productivity growth surprisingly reflects negative agglomeration effects. From the theoretical model it follows that congestion effects must have taken precedence over agglomeration effects during this period. Both agglomeration and congestion effects are dampened by job density in neighboring regions. Finally, policy simulations with the estimated model show that spatial concentration is more harmful to national productivity growth than spatial dispersion.