Wrist orthoses are the most common upper limb orthoses, being used by thousands of individuals each year to stabilize, immobilize, or support the wrist joint. Wrist orthoses achieve their function by altering the stiffness of the wrist joint (Figure 1-1). However, there is no quantitative understanding of how wrist orthoses affect wrist stiffness, and consequently, wrist orthosis development often relies on feel, intuition, or empirical heuristics rather than a methodical, quantitative approach. Because wrist movement control is dominated by wrist joint stiffness (Charles and Hogan 2011) a quantitative understanding of how wrist orthoses alter the stiffness of the wrist is imperative to the development of improved wrist orthoses with properties tailorable to the needs of the thousands of individuals who use them. In order to begin bridging this gap, our research characterized the stiffness of four common groups of wrist orthosis in two degrees of freedom: flexion-extension (FE) and radioulnar deviation (RUD) which are the degrees of motion most affected by wrist orthoses. We used a wrist robot to measure how twelve orthoses altered the passive wrist stiffness of twenty healthy subjects (three orthoses and five subjects per orthosis group). To perform these measurements we designed a unique wrist-mounting fixture (Figure 3-2) which allows the wrist robot to manipulate the hand inside an orthosis without interfering with orthosis motion (more accurately simulating the actual hand-orthosis interaction). Our results showed that (1) three out of four orthosis groups significantly altered the stiffness of the wrist joint, (2) orthoses in the same group are not generally significantly different than one another, and (3) there are important differences in stiffness between different orthosis groups. An interesting implication of our research is the result that in many cases orthoses with volar stays may be interchanged with orthoses with both volar and dorsal stays without significant changes in orthosis performance (Table 4-2). We anticipate this work will prove fruitful toward the future study of wrist orthoses' effects on wrist movement behavior and the future improvement of wrist orthosis design.