In the light of recent advances in the immunotherapy field for breast cancer (BC) treatment, especially in the triple‐negative subtype, the identification of reliable biomarkers capable of improving patient selection is paramount, because only a portion of patients seem to derive benefit from this appealing treatment strategy. In this context, the role of programmed cell death ligand 1 (PD‐L1) as a potential prognostic and/or predictive biomarker has been intensively explored, with controversial results. The aim of the present review is to collect available evidence on the biological relevance and clinical utility of PD‐L1 expression in BC, with particular emphasis on technical aspects, prognostic implications, and predictive value of this promising biomarker. Implications for Practice In the light of the promising results coming from trials of immune checkpoint inhibitors for breast cancer treatment, the potential predictive and/or prognostic role of programmed cell death ligand 1 (PD‐L1) in breast cancer has gained increasing interest. This review provides clinicians with an overview of the available clinical evidence regarding PD‐L1 as a biomarker in breast cancer, focusing on both data with a possible direct impact on clinic and methodological pitfalls that need to be addressed in order to optimize PD‐L1 implementation as a clinically useful tool for breast cancer management. 摘要 鉴于人们在乳腺癌 (BC)(特别是三阴性亚型)治疗的免疫治疗领域取得的最新进展，确定能够改善患者选择的可靠生物标志物至关重要，因为仅有一部分患者似乎可以从这种具有吸引力的治疗策略中获益。在这种背景下，程序性细胞死亡配体 1 (PD‐L1) 作为一种潜在的预后和/或预测生物标志物的作用已被深入探索，并取得了具有争议性的结果。本报告旨在收集关于BC中的 PD‐L1 表达的生物相关性和临床实用性的有效证据，特别强调了技术因素、预后意义以及这种前景可期的生物标志物的预测价值。 实践意义:鉴于针对乳腺癌治疗的免疫检查点抑制剂的试验结果前景可期，程序性细胞死亡配体 1 (PD‐L1) 在乳腺癌中的潜在预测和/或预后作用已得到人们越来越多的重视。本综述为临床医生提供了关于 PD‐L1 作为乳腺癌生物标志物的现有临床证据的概述，重点关注可能对临床产生直接影响的数据和需要解决的方法学缺陷，从而对在临床上对乳腺癌管理十分有益的工具 PD‐L1 的应用进行优化。 This article presents a comprehensive review of the current evidence related to PD‐L1 testing in breast cancer and its use as a biomarker in prediction of response to immunotherapies.