Context: Shoulder external rotators are challenged eccentrically throughout the deceleration phase of throwing, which is thought to contribute to overuse injuries. To evaluate the effectiveness of intervention programs, as well as identifying deficits, reliable and responsive measures of isometric and eccentric shoulder external rotation are necessary. Previously, isometric measures have primarily tested a single position, and eccentric measures have not been found to have high reliability. Objective: To examine the between-days reliability of multiple-angle isometric and dynamic eccentric isokinetic testing of shoulder external rotation. Design: Repeated measures. Participants: 10 healthy subjects (age 30 ± 12 y, height 166 ± 13 cm, mass 72 ± 10 kg). Main Outcome Measures: Average isometric peak torque of shoulder external rotation at 7 angles was measured. From these values, the angle of isometric peak torque was calculated. Dynamic eccentric average peak torque, average total work, and average angle of peak torque were measured. Results: Between-days reliability was high for average peak torque during isometric contractions at all angles (ICC ≥ .85), as it was for dynamic eccentric average peak torque (ICC ≥ .97). The estimated angle of isometric peak torque (ICC ≤ .65) was not highly reliable between days. The average angle of peak torque from the eccentric testing produced inconsistent results. Average total work of dynamic eccentric shoulder external rotation was found to be highly reliable between days (ICC ≥ . 97). Conclusion: Aspects of force such as peak torque and total work in isometric and eccentric testing of the shoulder external-rotator muscles can be measured reliably between days and used to objectively evaluate shoulder strength and identify changes when they occur. Angle measurements of peak torque could provide insight into the mechanical properties of the posterior shoulder muscles but were found to be inconsistent between days.