Little is known about the relationship of individual factors to wellbeing in mental health staff. The systematic review conducted for this thesis found evidence of associations between individual factors and wellbeing outcomes in this population, and suggests that a mindfulness-based construct, psychological flexibility, is of particular interest. Three studies were conducted to examine psychological flexibility in staff of Crisis Resolution Teams (CRTs). Staff in 25 CRTs (n=723) completed questionnaires including measures of psychological flexibility and wellbeing at two time points 12 months apart. Service user satisfaction and service use data was collected for each team. The resulting data has been used to investigate: 1) the extent to which psychological flexibility predicts wellbeing in individual CRT staff; 2) associations between managersβ levels of psychological flexibility and staff wellbeing and psychological flexibility; and 3) associations between team-level psychological flexibility and service user satisfaction and service use. The results provide evidence that, in line with previous research in other populations, psychological flexibility predicts better wellbeing at the individual level in CRT staff (Coeff. -0.36, 95% CI -9.38 to -0.94, p<0.01). In a novel contribution to the field, manager psychological flexibility was also found to be positively associated with better staff wellbeing (Coeff. -0.31, 95% CI -0.60 to -0.03, p=0.03). An unexpected result was that average team-level psychological flexibility was associated with lower service user satisfaction (this may be due to methodological factors) (Coeff. -0.55, 95% CI -1.08 to -0.02, p=0.04), and was not associated with service use. The results contribute evidence about psychological flexibility in a group not previously studied. They demonstrate the benefits of multi-level research, and suggest further research is warranted to investigate the use of interventions to increase psychological flexibility at the individual and management levels in mental health contexts, and thus improve wellbeing in this important group of staff