Human embyonic stem cells (hES) are cells derived from the inner cell masses of blastocysts. These only exist in the first few days of the earliest stage of development.

These stem cells have two very specific qualities; their pluripotency, or ability to become any cell in the human body, and their ability to indefinitely replicate. The embryonic stem cell's ability to replicate makes these extremely useful cells for research and regenerative medicine since limitless numbers can be reproduced for science and medicine.