Purpose: Clinical guidelines’ (CGs) adherence supports high-quality care. However, healthcare providers do not always comply with CGs recommendations. This systematic literature review aims to assess the extent of healthcare providers’ adherence to breast cancer CGs in Europe and to identify the factors that impact on healthcare providers’ adherence. Methods: We searched for systematic reviews and quantitative or qualitative primary studies in MEDLINE and Embase up to May 2019. The eligibility assessment, data extraction, and risk of bias assessment were conducted by one author and cross-checked by a second author. We conducted a narrative synthesis attending to the modality of the healthcare process, methods to measure adherence, the scope of the CGs, and population characteristics. Results: Out of 8137 references, we included 41 primary studies conducted in eight European countries. Most followed a retrospective cohort design (19/41; 46%) and were at low or moderate risk of bias. Adherence for overall breast cancer care process (from diagnosis to follow-up) ranged from 54 to 69%; for overall treatment process [including surgery, chemotherapy (CT), endocrine therapy (ET), and radiotherapy (RT)] the median adherence was 57.5% (interquartile range (IQR) 38.8–67.3%), while for systemic therapy (CT and ET) it was 76% (IQR 68–77%). The median adherence for the processes assessed individually was higher, ranging from 74% (IQR 10–80%), for the follow-up, to 90% (IQR 87–92.5%) for ET. Internal factors that potentially impact on healthcare providers’ adherence were their perceptions, preferences, lack of knowledge, or intentional decisions. Conclusions: A substantial proportion of breast cancer patients are not receiving CGs-recommended care. Healthcare providers’ adherence to breast cancer CGs in Europe has room for improvement in almost all care processes. CGs development and implementation processes should address the main factors that influence healthcare providers' adherence, especially patient-related ones. Registration:: PROSPERO (CRD42018092884).