Induced pluripotent stem cells (iPS cells) have revolutionized stem-cell biology by enabling the reprogramming of somatic cells into a pluripotent state, offering new opportunities for disease modeling and regenerative medicine. While iPS cells were initially thought to be functionally equivalent to embryonic stem (ES) cells, recent studies reveal subtle but significant molecular and functional differences between the two. These differences arise from technical limitations in reprogramming and may affect their utility in research and therapy. Despite these differences, iPS cells remain a valuable alternative to ES cells, particularly for personalized medicine, as they can be derived from a patient's own cells, reducing the risk of immune rejection. The derivation of iPS cells involves the ectopic expression of key transcription factors, such as OCT4, SOX2, KLF4, and c-MYC, which reprogram adult cells into a pluripotent state. This process has been successfully applied to various cell types, including fibroblasts, neural stem cells, and pancreatic β cells, demonstrating the broad capacity to alter cellular identity. However, iPS cells and ES cells differ in their epigenetic and functional characteristics, such as the ability to generate all three germ layers, their differentiation potential, and their capacity to form teratomas. These differences are influenced by the cell of origin, the reprogramming method, and the genetic and epigenetic state of the cells. Assessing the pluripotency of iPS cells involves a range of functional assays, including in vitro differentiation, teratoma formation, and chimaera assays. These assays help determine whether iPS cells can generate all cell types of the body and contribute to the development of adult tissues. However, the results of these assays can vary depending on the reprogramming method, the cell of origin, and the genetic and epigenetic state of the cells. This variability highlights the need for standardized criteria to evaluate the pluripotency of iPS cells and their suitability for therapeutic applications. Despite these challenges, iPS cells offer significant advantages over ES cells, particularly in terms of their potential for personalized medicine and their ability to avoid immune rejection. However, the differences between iPS cells and ES cells, as well as the variability within iPS cell lines, underscore the importance of careful evaluation and standardization in their use for research and therapy. As the field of stem-cell biology continues to evolve, the development of more refined reprogramming techniques and the establishment of standardized criteria will be crucial for maximizing the potential of iPS cells in both research and clinical applications.Induced pluripotent stem cells (iPS cells) have revolutionized stem-cell biology by enabling the reprogramming of somatic cells into a pluripotent state, offering new opportunities for disease modeling and regenerative medicine. While iPS cells were initially thought to be functionally equivalent to embryonic stem (ES) cells, recent studies reveal subtle but significant molecular and functional differences between the two. These differences arise from technical limitations in reprogramming and may affect their utility in research and therapy. Despite these differences, iPS cells remain a valuable alternative to ES cells, particularly for personalized medicine, as they can be derived from a patient's own cells, reducing the risk of immune rejection. The derivation of iPS cells involves the ectopic expression of key transcription factors, such as OCT4, SOX2, KLF4, and c-MYC, which reprogram adult cells into a pluripotent state. This process has been successfully applied to various cell types, including fibroblasts, neural stem cells, and pancreatic β cells, demonstrating the broad capacity to alter cellular identity. However, iPS cells and ES cells differ in their epigenetic and functional characteristics, such as the ability to generate all three germ layers, their differentiation potential, and their capacity to form teratomas. These differences are influenced by the cell of origin, the reprogramming method, and the genetic and epigenetic state of the cells. Assessing the pluripotency of iPS cells involves a range of functional assays, including in vitro differentiation, teratoma formation, and chimaera assays. These assays help determine whether iPS cells can generate all cell types of the body and contribute to the development of adult tissues. However, the results of these assays can vary depending on the reprogramming method, the cell of origin, and the genetic and epigenetic state of the cells. This variability highlights the need for standardized criteria to evaluate the pluripotency of iPS cells and their suitability for therapeutic applications. Despite these challenges, iPS cells offer significant advantages over ES cells, particularly in terms of their potential for personalized medicine and their ability to avoid immune rejection. However, the differences between iPS cells and ES cells, as well as the variability within iPS cell lines, underscore the importance of careful evaluation and standardization in their use for research and therapy. As the field of stem-cell biology continues to evolve, the development of more refined reprogramming techniques and the establishment of standardized criteria will be crucial for maximizing the potential of iPS cells in both research and clinical applications.