Resumen
Livestock population and its structure is the starting point for all development strategies at both micro and macro level, reason for which accurate knowledge of it is one of the main objectives of official statistics in each state. The analysis aims to highlight the major changes produced after the 90s in terms of population of Bacau county and its structure, with time effects on economic and social activity. Both the 2002 and 2011 census, Bacau was top of the list with an absolute decrease of the population. This was a concern as it has emerged over time as a stable phenomenon. The main reason for the decrease was due to external migration of the population and secondly because of the internal migration flows of population moving towards large economic centers in the country and abroad that attract labor. As most of the migrants are young and active age groups, this phenomena has led to an aging population and to imbalances that can be observed in the age pyramid, with negative effects becoming larger in the future.