Many scientists have expressed concern that declining agricultural diversity threatens agricultural sustainability. We draw on the available literature to outline and examine mechanisms that reduce agricultural diversity and identify the at-risk attributes of agricultural sustainability. Using a three-pillar concept embodying ecological, social and economic dimensions, this article provides a comprehensive general assessment of the sustainability of agricultural systems. It pays particular attention to consequences for agricultural diversity and sustainability of the increasing dependence of agriculture on the market system and new agricultural technologies. As an illustrative example, it examines changes in the diversity and sustainability of Bangladeshi agriculture by applying a novel index of the diversity of cropping land use, an output decomposition method, and statistical techniques. Crop diversity in Bangladesh is very low and dominated by the cultivation of rice, which now depends very heavily on a limited number of high yielding varieties (HYVs). Higher rice yields in Bangladesh and seasonal changes in rice cultivation have resulted in land sparing, which make room for greater crop diversity. Nevertheless, Bangladesh’s food dependence on its rice output is very high and is critically dependent on groundwater irrigation. We recommend that Bangladesh consider increasing the diversity of its crops as a food security measure and as a hedge against a decline in its agricultural sustainability.