Excel 2011 :: Automatically Calculate Running Subtotal By Category

I am using Excel 2011 for Mac.

I am creating a workbook with multiple worksheets, one overview sheet and then one sheet for each month of the year.

In the monthly sheets I have a Category column for which I have created a drop-down menu of expense categories. Then I have an Expense Amount column where I enter the amount spent.

The Category column will not necessarily remain sorted by category because I will be entering the expenses as they come up throughout the month so the categories will be all mixed up, unless I manually sort them.

What I want to do is somehow automatically calculate a running subtotal by Category (that will update with each new entry) and simultaneously automatically transfer the running subtotal for each Category to a specific cell on the overview sheet.