Tendon injuries are common and a significant source of pain and disability. Tendon injuries are often treated with a variety of non-surgical (e.g., physical therapy) and surgical interventions, along with a reduction of normal physical activities as the tendon heals [1–3]. However, it is difficult to objectively determine when the tendon has healed sufficiently and has the functional capacity to return to normal activities. Previous research has documented the functional capacity of tendons by measuring their stiffness under in vitro [4, 5] and in vivo [6, 7] conditions, but the technologies used for measuring tendon stiffness are applicable only to in vitro conditions (e.g., mechanical testing systems) or require the highly invasive implantation of sensors (e.g., strain gauges) under in vivo conditions. Conventional imaging modalities (e.g., ultrasound, MRI) can monitor changes in the appearance of tendons over time, but these imaging modalities do not provide an objective, quantitative assessment of tendon healing. A technique that could reliably provide a non-invasive quantitative assessment of tendon mechanical properties in vivo would have significant clinical application as a tool for monitoring tendon changes as a result of injury, pathology, and/or treatment. Shear-wave elastography is a relatively new technology that has the potential to assess the functional capacity of healing tendons in vivo. This technique applies an acoustic radiation force via an ultrasonic beam to the tissue(s), and then utilizes an ultrafast (up to 20 kHz) imaging sequence to measure the speed of the shear waves that result from this applied force. Shear-wave propagation increases as tissue stiffness increases, thus allowing for the indirect estimation of tissue stiffness from shear-wave speed. This technology has been used primarily as a diagnostic tool to identify fibrous masses in breast tissue [8] and has also been applied to the liver, arteries, and muscle [9–12], but its use in tendon is relatively new. Shear-wave elastography has been shown to have good reproducibility in breast imaging [8], but the repeatability for assessing tendon stiffness is not known. Therefore, the objective of this study was to determine the in vitro and in vivo repeatability of shear-wave elastography for estimating tendon stiffness. We hypothesized that in vitro repeatability would be better than in vivo repeatability, and that repeatability would be no worse than “moderate” (i.e., ICC≥0.41) under both in vitro and in vivo conditions.