Contemporary Western spirituality with its main theme being the evolution of consciousness (i.e., psychospiritual development) on the one hand, and the ancient Eastern contemplative traditions on the other, are significantly interrelated. Also, the central theme of transpersonal psychology from its inception in the late 1960s to the beginning of the 21st century has been - its position as a liaison between the Western mainstream psychology and the Eastern spirituality. There has been an overwhelming interest in those spiritual traditions and many of their concepts such as healing, self-realization, expansion of consciousness, transformation, self-transcendence and immanence, and others. This study discusses the psychological and spiritual structures, functions, and developmental values expounded by both Yoga (e.g., subtle energy system) and Buddhism (e.g., nonduality, mindfulness). Those concepts significantly overlap with some contemporary transpersonal theories that focus on nondual states and stages, cartographies of consciousness, positive transpersonal emotions (e.g., joy and loving-kindness) and recent neuro-cognitive research. The author also investigates how the work of some of the forefathers of transpersonal movement such as William James, Roberto Assagioli, and Carl G. Jung, as well as its eminent founders such as Abraham Maslow, Stanislav Grof, and Ken Wilber, was influenced by the central tenets of Buddhism and Yoga as systems of philosophy/psychology. Furthermore, their mutual interdependence and manifestations are analyzed, while the findings of this study reveal some possible future developmental, therapeutic and clinical implications. Some of the offered postulates have a heuristic origin, while others are based on the literature review.