The stratospheric ozone depletion story begins with the invention of
chlorofluorocarbons (CFCs) by Thomas Midgley in 1930.  He formulated these
compounds to replace dangerous chemicals (ammonia, sulfur dioxide) that
were used in refrigeration.  Their lack of reactivity, which made them safe
for ordinary use, was the property that made them efficient carriers of
their chlorine compounds to the stratosphere where they could deplete
ozone. Recognition of this problem for the ozone layer in the 1970s led, in
little more than a decade, to the formulation of the Montreal Protocol on
Substances that Deplete the Ozone Layer in 1987.   The Protocol took effect
in 1989 and has now been signed by 193 countries.

I will review the science underlying our understanding of the ozone layer
and the impact of chlorine compounds on that ozone layer.  I will further
describe the basic provisions of the Montreal Protocol that will illustrate
some of the reasons that it has been such a successful international
agreement.  In response to the provisions of the protocol, the key
long-lived CFCs have been phased out of usage and replaced by shorter-lived
HCFCs that are now being replaced by HFCs.  The total amount of chlorine in
the stratosphere has ceased its increase, peaked, and is now beginning a
slow decline towards pre-1980 amounts.  Evidence is accumulating that the
ozone layer is responding; ozone decreases of the 1980s and early 1990s
have slowed or ceased.  I will conclude by discussing future issues of
detection and attribution of ozone recovery in a changing climate.