Abstract: The career of Silvester Gardiner, a colonial surgeon, druggist, and land speculator, provides a view of the interpersonal dynamics at work within the eighteenth-century Atlantic commercial system. His business records emphasize the role of expertise (knowing something rather than someone) in the processes of network formation and maintenance underlying the system’s day-to-day business transactions. Gardiner began his career in Boston as a surgeon in the 1730s, leveraged his surgical practice into a successful pharmaceutical trade, and then by 1760 began to speculate in land along the Maine frontier. All the while he retained close ties to London as he expanded his businesses across New England. Expertise played a central role throughout Gardiner’s professional life, and the form of expertise he exhibited evolved as he developed commercial experience beyond his surgical training. The concept of expertise thus emerges as a key explanatory variable for how and why Gardiner and others like him positioned themselves at the intersections of several colonial trades in the eighteenth-century Atlantic world. Examining such individuals provides insight into the versatile networks that facilitated the movement of goods, ideas, money, and people in the service of British overseas expansion.