Import a file from a subdirectory?

I have a file called tester.py, located on /project.

/project has a subdirectory called lib, with a file called BoxTime.py:

/project/tester.py
/project/lib/BoxTime.py

I want to import BoxTime from tester. I have tried this:

import lib.BoxTime

Which resulted:

Traceback (most recent call last):
  File "./tester.py", line 3, in 
    import lib.BoxTime
ImportError: No module named lib.BoxTime

Any ideas how to import BoxTime from the subdirectory?

EDIT

The __init__.py was the problem, but don't forget to refer to BoxTime as lib.BoxTime, or use:

import lib.BoxTime as BT
...
BT.bt_function()