A/B testing is a process of carrying out a test to determine which of two email campaign options works best. To conduct an A/B test, you need to create two versions of the same campaign and send them to a small portion of your email list. You then monitor the performance of the two campaigns to determine which one performs better. For instance, to A/B test your email subject lines, you should come up with two different subject lines for the same email, and then send these two emails to a small portion of your email list. You then analyze the open rates of the two emails to determine which subject line performs better.