The invention of the capsule in the nineteenth century—and subsequent improvements to it in composition and manufacturing—focused on gelatin as the sole or principal film-forming component of the capsule shell. This chapter focuses on the rheological requirements for capsule shell excipients, rheological instruments, and procedures intended to test the suitability of the excipients for capsule shell production, and rheological considerations relevant to the filling of hard and soft capsules. Rheological principles play a substantial role in every aspect of the manufacture of capsules—from the formulation and manufacture of capsule shells per se to the filling of hard or soft shell capsules. The sensitivity and utility of rheological test methods, irrespective of the chemical or molecular composition of the capsules or their contents, underscore the value of rheology for modern encapsulation techniques and processes. Finally, the advent of more sophisticated rheological instruments and software has put these methodologies within the reach of most formulators and technicians.