<jats:p><jats:italic><jats:bold>Context:</jats:bold></jats:italic> Physicians and clinicians need portable, efficient, and cost-effective assessment tools to determine the effectiveness of rehabilitation programs after knee injury. Progress in rehabilitation should be evaluated using valid and reliable measurement methods. <jats:italic><jats:bold>Objective:</jats:bold></jats:italic> To examine the test-retest reliability of portable fixed dynamometry (PFD), handheld dynamometry (HHD), and isokinetic dynamometry (IKD). In addition, the authors sought to examine the validity of PFD and HHD by comparing differences in peak torque of the knee flexors and extensors to that of the "gold standard" IKD. <jats:italic><jats:bold>Design:</jats:bold></jats:italic> Repeated measures. <jats:italic><jats:bold>Participants:</jats:bold></jats:italic> 16 healthy subjects (age 29.3 ± 7.2 y, height 167.4 ± 8.04 cm, mass 73.7 ± 20.0 kg). <jats:italic><jats:bold>Main Outcome Measures:</jats:bold></jats:italic> The dependent variables were trial (trial 1, trial 2) and instrument (IKD, PFD, and HHD). <jats:italic><jats:bold>Results:</jats:bold></jats:italic> Test-retest reliability was high for both PFD and IKD. However, fair to poor reliability was found for HHD. There were no differences in peak torque (Nm) between IKD and PFD. However, significant differences in peak torque were observed between IKD and HHD and between PFD and HHD. <jats:italic><jats:bold>Conclusions:</jats:bold></jats:italic> PFD provides reliable measures of strength and also demonstrates similar output measures as IKD. Its portability, ease of use, and cost provide clinicians an effective means of measuring strength.</jats:p>