This brief review summarizes some of the preclinical findings of studies aimed at assessing the efficacy and safety of the aminobisphosphonate alendronate (ALN) in preventing or restoring the bone loss caused by calcium or estrogen deficiency. Mode of action studies show that ALN localizes at sites of bone resorption and inhibits osteoclastic activity. In secondary hyperparathyroidism caused by calcium-deficient diets in the rat, ALN reduced the bone loss. For low doses, daily administration proved most efficient. In ovariectomized rats, ALN both prevented and reversed the bone changes produced by estrogen deficiency at oral doses equivalent to 0.1 mg/kg per day or higher, and also maintained the mechanical strength of vertebrae. In ovariectomized baboons, which show bone changes similar to those seen in ovariectomized women, ALN also prevented the increase in bone turnover and increased both bone volume and bone strength in vertebrae. In a comparative study between ALN and etidronate, we found that ALN was 1000-fold more potent in inhibiting bone resorption and had at least a 1000-fold higher safety margin with respect to inhibition of mineralization and osteomalacia.