@Mishelli there is nothing already built into Excel. So if this is in hindsight (you want to know if someone already looked at a file) you are out of luck. For future access monitoring you can create a macro to do that by having everything hidden and they have to supply a username and password or maybe just use the Microsoft credentials and then make everything visible except maybe the sheet that your macro is recording that info. Of course they have to enable macros and you would have to make sure you can save the workbook before you show it. You would also want to prevent them from making copies somewhere else since then the sheet may record it but you would still not know it. That all said there are File System utilities that can control and monitor access better.