This volume establishes the biopsychosocial model as an integrating theory for psychological psychotherapy, thereby illustrating a path towards a scientifically grounded and cross-school way of thinking and working. Based on the system-theoretically justified body-soul unity, simultaneous diagnostics and simultaneous therapy are employed. It is emphasized that biological, psychological, and life-world-related influencing factors are always effective in a parallel interconnection of system levels and through various communication channels. This opens up broader therapeutic concepts than the previously tradition-based treatment approaches.