I don't like the example Alan referred you to. It separates the dependent lists into multiple tables which is just plain wrong in a relational database and makes the process more complicated than it needs to be. In summary:
The RowSource for each dependent combo should be a query with criteria that points to the "previous" combo for criteria.
Combo2
Select ... From ... Where something = Forms!yourform!combo1;
Combo3
Select ... From ... Where something = Forms!yourform!combo2;
etc.
Then in the AfterUpdate event of each "controlling" combo, clear and requery "dependent" combos.
AfterUpdate for combo1
Me.combo2 = Null
Me.combo3 = Null
...
Me.combo2.Requery
AfterUpdate for combo2
Me.combo3 = Null
..
Me.combo3.Requery
If you have more combos, just continue the pattern.