Reference a cell in another numbers file.
You can not do this currently in Numbers,I know. This has been answered already, but hopefully it may change in the future. This feature would be a tremendously useful tool for rolling forward "year to date" spreadsheets while keeping their sizes manageable. I have one spreadsheet for each of the last ten years of our family finances, but each year I have to manually enter the BOY balance for the coming year when doing budgeting. Of course, during the end of November and into the first weeks of January as I am both budgeting for the new year and closing out the previous year, with balances in multiple checking, savings, investment and charge accounts it is hard to keep them in sync and can sometimes take months to balance an account because of the EOY and BOY amounts matching from to different files. My two cents anyway...