Assuming you're using the DATE datatype, any of these:
WHERE YEAR(DateField) IN (2011, 2012)
WHERE YEAR(DateField) BETWEEN 2011 AND 2012
WHERE DateField BETWEEN '20110101' AND '20121231'
If there's an index on the DateField, the last one will perform the best, as the use of the YEAR function precludes the use of the index.
If you're using DATETIME, the last one would become:
WHERE DateField >= '20110101' AND DateField < '20130101'