Background: The extensor carpi radialis longus (ECRL) and extensor carpi radialis brevis (ECRB) muscles have both been described as wrist abductors and extensors and used in tendon transfers. In order to understand how a muscle is engineered for specific functional requirements it is important to document its architectural characteristics. Fiber length, angle of pennation, placement of aponeuroses and tendons has not been previously documented in three-dimensions throughout the volume of the ECRL and ECRB. Since muscles work together with tendons to exert their effect on bones, a detailed understanding of this relationship is clinically important when considering tendon transfers. Purpose: To model and examine the three-dimensional structure of the extensor carpi radialis longus (ECRL) and brevis (ECRB), including fiber architecture, aponeuroses, and tendons. Method: To observe the fiber bundle arrangement throughout the muscle, multiple layers of fiber bundles and aponeuroses were dissected and digitized using the Microscribe 3D-X robotic arm. The muscles were reconstructed in 3-D using DANCE software. Following digitization in situ, the tendons were removed, sectioned at 10-mm intervals, and the circumference of each piece digitized. Results: ECRB and ECRL were modeled in 3-D, enabling visualization from all perspectives. ECRL consists of parallel fiber bundles that are longer in the superficial anterior part (86mm ± 14) and shorter(70mm ± 11) in the deep posterior part. Both muscles have relatively small angles of pennation. In contrast, ECRB has shorter fiber bundles (52mm ± 9.8) and is partially pennate. The medial aspect of ECRB has shorter fiber bundles than the remainder of the muscle belly. ECRL has a long tendon that has the largest cross-sectional area proximally, decreasing distally, while ECRB has a shorter tendon (ECRL 205mm versus ECRB 123mm) that increases in cross-sectional area from proximal to distal. Conclusion: ECRB and ECRL have distinctly different muscle architectural arrangements and tendon dimensions. This suggests that ECRL is designed for excursion, while ECRB may generate greater force and act as a stabilizer. These results add further understanding to planning tendon transfers and rehabilitation protocols.