The induction of pluripotency can be achieved by forced expression of defined factors in somatic cells. The established cells, termed induced pluripotent stem (iPS) cells, have pluripotency and an infinite capacity for self-renewal in common with embryonic stem (ES) cells. Patient-specific iPS cells could be a useful source for drug discovery and cell transplantation therapies; however, the original method for iPS cell generation had several issues that were obstacles to their clinical application. Recent studies have brought about various improvements for iPS cell generation and uncovered several characteristics of iPS cells. Here we summarize the current status of iPS cell studies, with a focus on the improved methods that can be used to generate iPS cells, and also refer to the future challenges.