Liana (woody vine) abundance height diameter and climbing mode were studied in the mature tropical moist forest on Barro Colorado Island Panama (BCI). Because lianas are capable of extended horizontal as well as vertical growth sample plots were 100—m2 cylinders extending from the ground up to the treetops. The plots were randomly located in areas representing different stages of regeneration following treefalls. In order to examine canopy lianas closely trees in or adjacent to the sample plots were climbed freehand or with the aid of mechanical rope ascenders. Lianas are abundant on BCI and play important roles in forest dynamics. A hectare of old—growth forest had 1597 climbing lianas distributed among 43% of the canopy trees. Trees with a least one liana has higher than random probability of having more than one liana and individual lianas connected an average of 1.56 canopy trees. In the understory 22% of the upright plants <2 m tall were lianas and depending on the species between 15 and 90% of these plants were vegetative offshoots (ramets) and not true seedlings (genets). Lianas were most abundant in recent treefall gaps and decreased in abundance with time since last disturbance. Trellis availability was found to be a major factor limiting liana access to the forest canopy. Experimental manipulation of supports and experimental planting of Dioclea reflexa seedlings revealed that trellises consisting of small diameter closely spaced supports are most abundant on the edges of treefall gaps. Tree and liana stems on the edges of treefall gaps provided a major pathway to the canopy for climbing plants. Trees carrying lianas suffered higher mortality rates and upon falling caused more other trees to fall than did liana—free trees. Few lianas died when their host tree fell and many grew back to the canopy using the abundant trellises that occur on the edges of treefall gaps. Tree sapling growth rates in treefall gaps and Luehea seemannii growth rates in the canopy were slower where lianas were abundant.