Objectives: Deliberative processes in Health Technologies Assessment (HTA) result in recommendations that determine the reimbursement of medicines, diagnostics or devices. These processes are governed by explicit criteria, but are also influenced by implicit factors. The objective of this work was to identify the implicit factors influencing HTA deliberative processes in five European countries (France, Germany, Italy, Spain and the UK). Methods: A systematic review of literature published between 2009 and 2019 was conducted. The search was performed in Pubmed, The Cochrane Database of Systematic Reviews, Google Scholar and Center for Reviews and Dissemination. The ISPOR database was searched manually. Results: Out of 100 eligible publications, eight articles were selected for data extraction and analysis. The implicit factors in the HTA deliberative process most frequently mentioned in the identified literature are value judgments, biases, preferences and subjectivity. Five out of the eight articles highlight the need to further improve the transparency of the process, and three provide recommendations on how to address the influence of implicit factors on the HTA deliberative process through a framework. Conclusion: Even in countries with a long HTA history, evidence on implicit factors is scarce. Some methods have been recommended for addressing these factors. Further research is required to characterize the implicit factors in the HTA deliberative process at a country level and explore potential ways to mitigate the influence of these factors on the HTA deliberative process.