Embryonic stem (ES) cells, derived from the inner cell mass of the mammalian blastocyst, can continuously proliferate in an undifferentiated state and can also be induced to differentiate into a desired cell lineage. These abilities make ES cells an appealing source for cell replacement therapies, the study of developmental biology, and drug/toxin screening studies. As compared to mouse ES cells, human ES cells have only recently been derived and studied. Although there are many differences in properties between mouse and human ES cells, the study of mouse ES cells has provided important insights into human ES cell research. In this review, we describe the advantages and disadvantages of methods used for human ES cell derivation, the expansion of human ES cells, and the current status of human ES cell differentiation research. In addition, we discuss the endeavor that scientists have undertaken toward the therapeutic application of these cells, which includes therapeutic cloning and the improvement of human ES cell culture conditions.