How do I limit the contents of one combo box based on the selection in another?

You can link combo boxes in a hierarchical way so that the choice made in each one limits the values displayed in the next. For example, on an order form, you can have a combo box for suppliers and a combo box for products. When you choose a supplier in the first combo, the second displays only products available from that supplier.

To do this, the second combo box's Row Source property must be set to a query or an SQL statement that refers to the first combo box. If you use an SQL statement (or a query in SQL view), the reference is placed in the WHERE clause. In the above example, the cboProducts combo box might have the following RowSource property:
SELECT Products.ProductName FROM Products WHERE Products.Manufacturer = Forms![MyForm]![cboSuppliers].Value

If you use a query in design view, the reference goes in the Criteria line. In the given example, the query would select the ProductName field, and the Criteria for that field would contain:
Forms![MyForm]![cboSuppliers].Value

There's one more step. You need the second combo box to rebuild its list whenever a new selection is made in the first combo box. To do that, create an AfterUpdate event procedure for the first combo box, and enter the following statement:
cboProducts.Requery

This technique extends well to third- and higher-level combo boxes. Say, in the given example, products are available in different styles. You could add a third combo box, cboStyles, referring to Forms![MyForm]![cboSuppliers].Value and Forms![MyForm]![cboProducts].Value. In this case, cboProducts would need an AfterUpdate event to execute the cboStyles.Requery method. You'd also want to requery cboStyles in cboSuppliers' AfterUpdate event.