Although it is difficult in fully clarifying its mechanisms and effects, Deqi still can be considered as an instant "sign" of acupuncture response of the patient and acupuncturist, which has a significant value in clinic and research. This paper aims to take a history trace to the development of Deqi theory, understand the connotation of Deqi based on Chinese medicine theory, and establish an evaluation methodology accordingly. We believe that Deqi is not only the needling sensation, but also the perception of changes of qi (') flowing of the patient elicited by needling on acupoints. The signs of Deqi include the patient's subjective perception (needling sensation), the objective physiological changes (common referred to the skin redness around the acupoints and the response of brain), and the acupuncturists' perception. Although Deqi is essential for attaining the effect, it may not be the necessary sign of the ideal efficacy. It is found that the characteristics of Deqi sensations, Deqi's intensity, time duration, and the propagation will all affect the efficacy. Thus, acupuncturists should pay attention to elicit and control Deqi state, which is also the key point in modern research on the therapeutic implications of Deqi.