This note first documents the facts around the sustainability of competitive advantage. It then observes that the demise of a previously successful strategy, in the first instance, often comes from some change in the external environment. It, therefore, characterizes the types of change that can lead to strategic decline. But external change alone should not mean the end of superior performance since the skilled strategist ought to be able to adapt to such changes. The final part of the note looks inside the firm to examine why managers often fail to respond adequately to external threats and explains why it is valuable to study the causes of strategic decline.