Presented here is a newly developed Boundary Effect Detection (BED) method for pinpointing locations of small damage to structures using Operational Deflection Shapes (ODSs) measured by a scanning laser vibrometer. The BED method requires no model or historical data for locating structural damage. It works by decomposing a measured ODS into central solutions and boundary-layer solutions by using a sliding-window least- squares curve-fitting technique. For high-order ODSs without damage, boundary-layer solutions are non-zero only at structural boundaries. For a damaged structure, because damage introduces new boundaries, its boundary-layer solutions are non-zero at damage locations as well as its original boundaries. At a damage location, the boundary-layer solution of slope changes sign, and the boundary-layer solution of displacement peaks up or dimples down. The theoretical background is shown in detail. Experiments are performed on several different structures with different damages, including surface slots, edge slots, surface holes, internal holes, and fatigue cracks. Experimental results show that this damage detection method is more sensitive and reliable for locating small damage than other dynamics-based methods using curvatures or strain energies.