Interviewers will often call previous employers for references when screening applicants, so they may already know why you left your last job. They want to hear from you your reason for leaving. It might be that you left because of a career change, you wanted a shorter commute, the ownership changed, the company downsized, or you were fired. Whatever your answer, be sure you do not speak negatively of your previous supervisor or coworkers, which will make you look bad in the interviewer's eyes. If you were fired, answer in a way that demonstrates your integrity and professionalism.