This article introduces various definitions and criteria for the astronomical phenomena of "encroachments" (close lunar and planetary conjunctions) in pre-modern China. With improvements in observations and mathematical astronomy, the standard of encroachments began to undergo many changes leading to more precise explanations of the phenomena. Before the adoption of Huihui Lifa (Islamic-Chinese calendrical astronomy), traditional Chinese methods could not predict the phenomena of encroachments, and records of encroachments were mainly based on actual observations. These records abound in Chinese dynastic histories, and they play an essential role in the interpretation of astrological omens. In the first half of the seventeenth century, the prediction of encroachments became an effective approach for examining the accuracy of different elements of calendrical astronomy besides solar and lunar eclipses. In the meantime, with the introduction of western astronomical knowledge into China, people had a better understanding of the principle of encroachments, and they began to question its rationality in astrology. Moreover, new knowledge of encroachments also brought new insights and inspired some philosophical discussions on the structure of the cosmos. However, these new astronomical methods still served the old astrology, due to the continued requirements that astronomers interpret astrological omens.