Viability means "the quality or state of being able to live, grow and develop." Limit of viability has been changed along with the advancement of perinatal and neonatal medicine. The viability limit defined in the Japanese Motherhood Protection Act was amended from 24 to 22 completed weeks of gestation in 1991 based on the survival rate of extremely preterm infants. Survival rates of infants at 22 and 23 weeks' gestation born between 2002 and 2004 in Japan were 31% and 56%, respectively. Though medical data are the most important means to define viability, socio-economical and ethical factors should be also considered, especially when we discuss the group of marginally viable infants. We think there are two different categories of viability limits, one from biological survival limit and another from social agreement of providing active medical intervention. Currently in Japan, the former viability definition based on purely medical data is 22 completed weeks and the latter definition is the social consensus of about 24-28 weeks' gestation.