Adhesives play an indispensable role in the manufacture of many modern engineered wood products, such as plywood, oriented strandboard, laminated veneer lumber, and cross-laminated timber. However, the development of new and improved wood adhesives is hindered by an incomplete understanding of the performance of wood–adhesive bondlines in these products. The wood adhesive development process is inefficient as new testing is often needed for changes made to any of the dozens of variables associated with the adhesive, wood, bonding processes, and service life that can affect bondline performance. An improved understanding of adhesive penetration in the wood interphase has been identified as a key to developing new and improved wood adhesives, especially for moisture durability. Adhesive penetration includes both flow of adhesives into wood micron-scale voids and infiltration into the polymer components of the wood cell wall layers. In this work, X-ray computed tomography (XCT) and X-ray fluorescence microscopy (XFM) were used to study adhesive flow and infiltration. The results provide new insights that are needed in the development of improved models that are capable of predicting the performance of adhesive bondlines, as process variables change. This should accelerate efforts to develop new and improved wood adhesives.