The nineteenth century was a crucial period of development for toxicology in Britain, during which several great toxicologists rose to prominence. These toxicologists facilitated the creation of communities and intellectual networks of toxicology that operated throughout Scotland and England. Despite this crossover, previous scholarship has tended to focus on either Scotland or England, rather than both together. In assessing toxicology in both countries, this thesis aims to provide a new perspective on toxicology as it was used in criminal investigations. By tracking the development of toxicology over the nineteenth century in Scotland and England, this thesis examines how different aspects of toxicology were performed both within and across national borders. Despite the pressures exerted by the different national frameworks of law and education in each country, toxicology developed along a similar trajectory in each, a process that is examined using two toxicologists as focal points. Robert Christison and Alfred Swaine Taylor were the most eminent toxicologists in Scotland and England respectively, and through their positions of authority had enormous influence over the growing body of toxicology practitioners. This thesis uses them to examine the communities and intellectual networks that emerged within the body of toxicology practitioners, and changes in toxicological practice. This approach reveals the interdependency of expert toxicologists and ordinary practitioners across Britain. The writings of Christison, Taylor, and other toxicologists in medical journals and textbooks are the most important sources in this thesis, because they allow reconstruction of the situations and reactions of ordinary medical men, who made up the bulk of toxicology practitioners.