I have an automated process that generates and runs the alter index rebuild statements for each index in my database based on fragmentation. It has been running fine for over one year. I am now looking into utilizing the SORT_IN_TEMPDB options since some of these tables are large. According to the documentation, you can utilize this option with the ONLINE = ON. I tried some tests on a stand alone db monitoring the tempdb usage. I set the trigger levels to capture tempdb data from the dmv's using sys.dm_db_file_space_usage when the freespace dropped below 98% and it did not change. I searched the web and came across this technical Article, SQL Server 2005 Online Index Operations. Reading the article it basically stated that using online operations would not trigger any activity in TEMPDB event if the SORT_IN_TEMPDB option was set. Does anybody know why this is this way. Why allow the combination of ONLINE and SORT_IN_TEMPDB both ON if it does nothing? The article mentioned that it does utilize TEMPDB when processing an non-clustered index with ONLINE = OFF. This I was able to verify.