Scientists have long been interested in measuring external qi (EQ or wai qi) during qigong healing, and have produced a large body of literature over the past 20 years. This paper reviews the major research on measuring EQ in China and tries to help other researchers to get a picture on what has been done so as to eliminate the simple replication of already verified results. Starting with the historical background of EQ studies in China, this paper analytically reviews the major studies of EQ effects from five different categories of detectors: 1) physical signal detectors; 2) chemical dynamics methods; 3) detectors using biological materials; 4) detectors using life sensors; and 5) detectors using the human body. The focus is on the pros and cons of each detector. These studies documented some important correlates of EQ process or qi healing, which cannot be explained by psychological effect or the known biological processes. Even though the extant literature suggests that intent plays a critical role in the effect or characteristics of EQ we know little about its role in EQ effect and its relationship with qigong healing from these experiments. These studies have confirmed the existence of measurable EQ effects from various perspectives; however, none has really revealed the primary nature of EQ or how EQ healing works. Given the fact that qigong therapy is based on the dialectic view of two interdependent spheres, while modern science and medicine is based on the reductionist view of one material world, it is recommended that future studies should use more biological or life-sensor detectors to increase our understanding of the healing potentials of qigong, instead of stay at the level of verification of signals. New methodologies, new theories, and new perspectives are urgently needed for further understanding what qigong is and how EQ healing works.