The optimum water content for tillage has been defined as the water content at which production of small aggregates is greatest and production of large aggregates or clods is least. Recent work on the optimum water content for tillage and on the soil structures produced by tillage has been based on the water retention characteristics of soils (plotted as gravimetric water content against the natural logarithm of the pore water suction in hPa). It was assumed that the van Genuchten water retention equation, which has a unique inflection point, provides an adequate fit to the water retention data. When this equation has been fitted to experimental data, it has been found that the optimum water content for tillage is given by the water content at the inflection point of the fitted curve, and that the size distribution of clods and aggregates produced by tillage is related simply to the slope, S, of the curve at the inflection point. Although this procedure is useful for prediction purposes, it does not explain why the inflection point is the optimum water content for tillage. A more detailed examination of water retention data has shown that most soils have pore size distributions that are bi-modal. That is, there are two distinct peaks of pore size corresponding with the textural and structural pore spaces that usually occur in the range of water suctions used in traditional water retention studies (10-15,000 hPa). In this paper, results of tillage experiments are re-examined in terms of the bi-modal pore size distribution of soils. It is shown that the optimum water content for tillage occurs when the matrix porosity is saturated, but the structural porosity is drained (i.e. is air-filled). The mechanism of crumbling of soil during tillage is discussed in terms of the expansion, elongation and joining-up of pre-existing, air-filled micro-cracks. These comprise the air-filled portion of the structural porosity. It is concluded that taking account of the bi-modal nature of the soil pore size distribution leads to an improved understanding of the physical basis of tillage.