Since 1997 SIRGAS (Geocentric Reference System for the Americas) is acting for establishing a unified SIRGAS Vertical Reference System (SVRS) in Latin Ameri- ca and the Caribbean. The standards, procedures and activities aiming the definition and realization of the SVRS are now directed to meet the modern international requirements of integration into a Global Vertical Reference System (GVRS) to support the needs for Earth Observation Systems. These statements imply in a uni- fication of the existing National Vertical Reference Systems (NVRS), each one with its respective Frame (NVRF) and Vertical Datum (NVD) realized in the geopoten- tial space. The study of new possibilities for connecting NVRS in a global basis is in the context of the International Association of Geodesy (IAG) main activities. However, most of modern requirements are not immediately applicable for building the SVRS because the differences in the conception and realization of each NVRS in the SIRGAS region. In this work an overview about the status of NVRS, NVR Fand NVD in South America is presented. The most important characteristics for unifying NVRS in a global context as well as the main related problems are pre- sented. Some emphasis is given on the activities of SIRGAS-Working Group III directed for facing the integration problems in South America. The poor data availability, unknown discrepancies among NVRS, different data collecting and processing strategies for establishing the networks, and other error sources are discussed. The possibilities coming from new strategies and modern tools are dis- cussed mainly by considering new missions and global data basis of Geodesy.