Background and Objective: The novel antibody-drug conjugate (ADC) analogues are emerging in the field of human epidermal growth factor receptor 2 (HER2) low and ultralow breast cancer, and accurate screening of patients with HER2 low expression poses a serious challenge to both pathological testing processes and diagnostic assessment. The purpose is to provide some suggestions on how to correctly understand HER2 low and ultralow breast cancer. Methods: This paper explores the clinicopathological features, immunohistochemical staining and its heterogeneity of HER2 low and ultralow expression breast cancer through literature search. Key Content and Findings: This article mainly elaborates on four aspects: the differences in clinical pathological characteristics and prognosis between HER2 immunohistochemistry (IHC) low expression and ultralow expression, which are suitable for screening new detection methods for HER2 ultralow expression and HER2 unstained expression, the heterogeneity of HER2 low expression and ultralow expression, and the factors affecting the pre detection process of HER2 low expression and ultralow expression. Conclusions: For the precise pathological diagnosis of HER2 ultralow expression and low expression, further standardization of the definition of HER2 low expression is needed to ensure consistency and accuracy in clinical practice. At the same time, it is recommended to reevaluate the HER2 status in the event of disease recurrence or metastasis, even if the previous test result is HER2-0. On the other hand, it is recommended to further improve and optimize detection techniques and methods, reduce false negatives and false positives, and enhance the sensitivity and specificity of detection.