=sumif(Sheet1!A1:A5,A1,Sheet1!B1:B5)
Sheet1A1:Foo B1:1A2:Bar B2:2A3:Foo B3:3A4:Bar B4:4A5:Baz B5:5
Sheet2A1:Foo B1:=sumif(Sheet1!A1:A5,A1,Sheet1!B1:B5)
Not sure where the problem is.