Adoptive transfer of natural killer (NK) cells has been demonstrated as an efficacious and safe treatment option in patients with cancer. For clinical use, these potent immune cells can be directly isolated or generated from several sources using a variety of protocols. Increasing evidence indicates that NK cell products prepared from hematopoietic stem and progenitor cells isolated from umbilical cord blood have many advantages. These include a highly cytotoxic activity against hematologic and solid malignancies, a minimal risk of adverse events, including graft-versus-host disease, and wide availability and applicability. The objective of this review is to provide an oversight of the various NK cell products and to discuss their advantages and disadvantages for cancer immunotherapy.