This study presents the hydrogeological characteristics and flow of formation waters in the post-Paleozoic succession of the Llanos basin, a mainly siliciclastic foreland sub-Andean sedimentary basin located in Colombia between the Cordillera Oriental and the Guyana Precambrian shield. The porosity of the sandy formations is generally high, in the range of 16-20% on average, with a trend of de-creasing values with depth. Permeabilities are also relatively high, in the 10² and 10³ md range. The salinity (total dissolved solids) of formation waters is generally low, in the 10,000-20,000 mg/L range, suggesting that at least some strata in the basin have been flushed by meteoric water. The shaly units in the sedimentary succession are weak aquitards in t e eastern and southern parts of the basin, but are strong in the central-western part. The pressure in the basin is close to or slightly subhydrostatic. The underpressuring increases with depth, particularly in the central-western area. The flow of formation waters in the upper units is driven mainly by topography from highs in the southwest to lows in the northeast. Local systems from the foothills and from local topographic highs in the east feed into this flow system. The flow of formation waters in the lower units is driven by topography only in the southern, eastern, and northern parts of the basin. In the central-western part, the flow is downdip toward the thrust-fold belt, driven probably by pore-space rebound induced by erosional unloading, which also is the cause of underpressu ing. Hydrocarbons generated in the Cretaceous organic-rich, shaly Gacheta Formation probably have migrated updip and to the north-northeast, driven by buoyancy and entrained by the topography-driven flow of formation waters; however, the downdip flow of formation waters in CretaceousOligocene strata in the central-western part of the basin could have created conditions for hydrodynamic entrapment of hydrocarbons.