Defects are crucial to the chemistry of metal-organic frameworks (MOFs), determining their sorption behavior, catalytic activity and even their thermal properties. The effect of defects on the properties of MOFs depends not just on the concentration of defects but their distribution. In this talk, drawing on analogies with defective oxides, I will show some examples of the radically different ways high defect concentrations can be accommodated in hafnium and zirconium MOFs, from defect nanodomains to 2D nanosheets, and the consequences of this for their reactivity and properties.