On-line marketers are fast pushing the boundaries of the field through a combination of creative content, the use of in-depth market knowledge, and the deployment of interactive and analytical technological tools. Targeted on-line marketing has emerged as a viable form of reaching customers on the Internet. In addition to providing a cheap and effective way of reaching qualified and interested end customers, it offers several other advantages in the way marketers can establish long-term customer relationships, leading ultimately to retention and loyalty. In this paper, we offer an introduction to the phenomenon and describe typical business models for targeted marketing online. We do this through a comparative case analysis of Netcentives, Inc., a leading integrator of interactive media technology; Agency.com, a leading interactive media agency and NetPerceptions, Inc., a provider of collaborative filtering software. We then outline some of the challenges and opportunities in this new medium going forward, including issues related to consumer privacy and the changing nature of intellectual property.