Physical activity (PA) benefits both physical and mental health. Total PA encompasses four domains -- utilitarian, transportation, occupational, and leisure-time (LTPA)-- and engagement in each domain is inconsistently associated with health outcomes. For example, LTPA is positively associated with both physical and mental health; whereas, occupational PA is associated with better physical (but not mental) health. This thesis used self-determination theory to investigate associations between motivational needs for autonomy, competence and relatedness and domains of PA among workplace wellness physical activity program (WWPAP) participants and non-participants. Three distinct samples (WWPAP participants, follow-up WWPAP participants, and non-participants) completed separate electronic surveys that measured PA, motivations for PA, and socio-demographic characteristics; satisfaction of motivational needs also was measured among follow-up participants. Differences in characteristics between participants and non-participants suggest that the WWPAP is predominately serving people of higher education who hold faculty positions, and therefore is not serving the entire employee population. 82% of non-participants and 98% of follow-up WWPAP participants met the PA recommendations for Americans to engage in 150-minutes of moderate intensity PA every week. Total metabolic equivalent (MET) minutes of PA were lower among WWPAP participants as compared to non-participants. Higher total Met minutes of PA among non-participants largely was due to greater occupational PA. WWPAP participants had significantly greater MET minutes of LTPA (such as that provided by WWPAPs), but not enough to compensate for much lower occupational PA. Motivational needs for autonomy and competence were higher among WWPAP participants than non-participants and, among follow-up WWPAP participants, need for autonomy regarding PA was not satisfied. Non-participants, who primarily perform occupational PA, may not reap the potential positive mental health benefits achieved with LTPA like the opportunities available through WWPAPs. Differing characteristics, motivations, and behaviors of employees who voluntarily participate in a WWPAP and those who do not may inform the development of future workplace wellness programs to improve the participation of all employees. Therefore, workplace wellness programs should also address non-PA options when creating programming. Future research should examine how participation in different components of the wellness program affects employee health and wellbeing.