Non Matching Records in Two Tables

I need some help with a bit of SQL. I have two tables. I want to find records in one table that don't match records in
anothertable based on a common column in the two tables. Both tables have a column named 'key100'. I was trying
somethinglike:<br /><br /> SELECT count(*)<br /> FROM table1, table2<br /> WHERE (table1.key100 != table2.key100);<br
/><br/> But the query is very slow and I finally just cancel it. Any help is very much appreciated.<br /><br /> -Ken