=index(a:a,match(d1,b:b,0))

You may want to read Debra Dalgleish's notes:
http://www.contextures.com/xlFunctions02.html (for =vlookup())
and
http://www.contextures.com/xlFunctions03.html (for =index(match()))

Graham Haughs wrote:

It is probably better to explain what I am trying to do rather than show
the INDEX, MATCH and other permutations I have been trying and failing
miserably with. I have for example two columns of Data A & B. The values
in column B are unique so are no repeated anywhere else in the column.
If I have a value in cell D1 for example, say 31, I want to find that
value in Column B, if it exists, then return the value in Column A in
the same row. I hope this is understandable and if so I would be really
grateful for any ideas.

Regards
Graham Haughs
Turriff
Scotland

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Dave Peterson