In mammalian testis, numerous sperms are produced persistently for a prolonged period, which plays an essential role for the continuity of life. It is generally considered that ‘stem cells’ that are both capable of self-renewal and differentiation supports the continuity of spermatogenesis. ‘Stem cells’ are also considered to be crucial not only for steady state spermatogenesis but also for regeneration after damage or transplantation. However, it is still largely to be elucidated what is the cellular nature of the ‘stem cells’ and how they behave in the testis to give rise to the different aspects of stem cell functions. We have been investigating this issue by means of live-imaging and pulse-labeling. These have shown us, slowly but steadily, how the stem cell functionality is achieved in the testis. It has been suggested that, rather than a very limited number of defined ‘stem cells’, extended population of undifferentiated cells with different degree of self-renewing and differentiating abilities compose the functional stem cell compartment. Interestingly, it is largely influenced by the tissue situation (steady-state, regenerating after damage, or post-transplantation) how much the cell population is recruited to the active population with the stem cell functionality. These finding may expand our view regarding the definition of ‘stem cells’ in the mouse spermatogenesis.