This is the scenario:
I have 2 comboboxes, cboDepartment and cboEmployee.
When I select an item in cboDepartment I want cboEmployee to only show employees in the relevant department.

Does anyone have an example of doing this in WPF?

I won't give you an example, but it's fairly simple.

You should specify a data source in your XAML (that is an ObservableCollection in your code) and set your cboEmployee to be bound dynamically to this data source.

When you change your department, you should update your data source. WPF will see the change and update your combo box.

Try and write up some code to this affect and if you get stuck on any of it, paste what you did and I'll have a look.