Assumptions constrain the vision and ability of health care systems throughout the world to achieve unprecedented levels of performance. Leaders who want to accelerate improvement should themselves question these assumptions and provide a context in which others can do so. Six current assumptions are particularly troublesome and particularly worthy of careful reconsideration: (i) that future performance levels will be approximately the same as current levels (rather than believing in the pervasive possibility of breakthrough); (ii) that measurement induces improvement (rather than emphasizing leadership of change as the key to improvement); (iii) that professional and organizational boundaries must be carefully preserved (rather than reducing those boundaries); (iv) that patients are passive and caregivers are active (rather than working from strong notions of equal partnership); (v) that traditional forms of space and equipment are well designed (rather than valuing fundamentally new designs); and (vi) that medical care operates in an environment of scarcity (rather than noticing and employing what it has in abundance).