Backscatter communication (BC) has been introduced as a cutting-edge technology due to low cost and ultra-low-power consumption. However, its openness and broadcast nature bring severe security threats and vulnerabilities to BC systems. Physical layer authentication (PLA) is viewed as a supplement to cryptography-based authentication on backscatter devices (BDs) to provide enhanced security for BC by ensuring the eligibility of communication parties. Through feature extraction from signals as the fingerprint of a BD for authentication, computation and energy consumption at the resource-limited BD can be greatly reduced due to the avoidance of complex cryptography calculations. Although many existing works are devoted to exploiting PLA on BDs in BC systems, a technical overview of PLA in BC is still missing. Based on a comprehensive analysis of the security threats in BC systems, this article gives a review on PLA schemes in BC by employing a set of proposed criteria to analyze their performance. Based on the review, open issues are pointed out, with valuable suggestions on future research directions.