Abstract BP operates the Cusiana volatile oil field and the Cupiagua gas condensate field in the Andean Mountains foothills province of Colombia (Fig. 1.). In 1992, a permanent seismic network of ten surface stations was installed in Cusiana and Cupiagua to obtain data for seismic hazard models necessary for the design of field infrastructure. The network is now in its sixteenth year of continuous operation. Currently, an average of 1000 microseismic events per month is recorded. The resulting seismological dataset is of high quality covering a range of seismic magnitudes down to about 1.0 on the Ritcher scale. Over time, the Cusiana-Cupiagua Seismic Network (CCSN) has been used for different purposes. During the past few years, it has become increasingly evident that the network and its data is an invaluable asset for evaluation of conditions relevant to production/injection operations within the reservoirs and adjacent areas. From the reservoir characterization and production operation standpoints, microseismic monitoring (also known as passive seismic) has had two main applications in Cusiana and Cupiagua: (i) to identify production/injection induced high transmissibility pathways and their temporal variations, and (ii) to image the orientation, extension, complexity, and temporal growth of hydraulic fractures. This paper is focused on the first of these applications: how microseismicity has been used as a surveillance tool to track movement of reservoir fluids away from the wellbore. A short description of the seismic network is provided. Then, the methodology for data interpretation is discussed. Finally, partial results are presented showing how microseismicity monitoring is being applied to: (i) assess transmissibility changes due to stress and pore pressure changes through time, (ii) identify potential reactivation of pre-existing weak planes, and (iii) calibrate numerical models to improve history matches. Analysis of the data shows a strong correlation between reservoir dynamics and production induced microseismicity in Cusiana and Cupiagua Fields with great potential as a surveillance tool for improved reservoir characterization and management.