This is a question that makes most people go weak in the knees during an interview. Interviewers commonly employ this question for a number of reasons:

To see how well you think on your feet

To detect any negativism about yourself or former employers.

This is also a way for the interviewer to obtain your honest appraisal of your abilities. There is a very simple and easy way to handle this question: candidly and honestly. While you should not throw caution to the wind when you give your answer, your interview preparation session will have given you ample opportunity to develop a terrific response. No one is perfect; we all have strengths and weaknesses.