This book constitutes the refereed proceedings of the 12th International Conference of the CLEF Association, CLEF 2021, held virtually in September 2021.

Full Papers.- Audio Features, Precomputed for Podcast Retrieval and Information Access Experiments.- A Hybrid Approach for Stock Market Prediction Using Financial News and Stocktwits.- Dataset Creation Framework for Personalized Type-based Facet Ranking Tasks Evaluation.- SubjectivITA: An Italian Corpus for Subjectivity Detection in Newspapers.- Evaluating Elements of Web-based Data Enrichment for Pseudo-Relevance Feedback Retrieval.- End-to-end Fine-grained Neural Entity Recognition of Patients, Interventions, and Outcomes.- Comparing Traditional and Neural Approaches for detecting Health-related Misinformation.- Towards the evaluation of Information Retrieval Systems on evolving datasets with pivot systems.- A Multi-Task Learning Model for Multidimensional Relevance Assessment.- Skill Extraction for Domain-Specific Text Retrieval in a Job-Matching Platform.- Linguistic Uncertainty in Clinical NLP: a Taxonomy, Dataset and Approach.- Best of 2020 Labs.- Self-Calibrating Neural-Probabilistic Model for Authorship Verification Under Covariate Shift.- Priberam at MESINESP Multi-label Classification of Medical Texts Task.- Herbarium-Field Triplet Network for Cross-Domain Plant Identification.- BERT-based Transformers for Early Detection of Mental Health Illnesses.- Dowsing for Math Answers.- Overviews 2021 Labs.- Overview of ARQMath-2 (2021): Second CLEFLab on Answer Retrieval for Questions on Math.- Overview of BioASQ 2021: The ninth BioASQ challenge on Large-Scale Biomedical Semantic Indexing and Question Answering.- The CLEF-2021 CheckThat! Lab on Detecting Check-Worthy Claims, Previously Fact-Checked Claims, and Fake News.- Overview of ChEMU 2021: Reaction Reference Resolution and Anaphora Resolution in Chemical Patents.- Overview of the CLEF eHealth Evaluation Lab 2021.- Overview of eRisk 2021: Early Risk Prediction on the Internet.- Overview of the ImageCLEF 2021: Multimedia Retrieval in Medical, Nature, Internet and Social Media Applications.-Overview of LifeCLEF 2021: an Evaluation of Machine-Learning based Species Identification and Species Distribution Prediction.- Overview of LiLAS 2021 - Living Labs for Academic Search.- Overview of PAN 2021: Authorship Verification, Pro ling Hate Speech Spreaders on Twitter, and Style Change Detection.- Overview of SimpleText 2021 - CLEF Workshop on Text Simplification for Scientific Information Access.- Overview of Touch e 2021: Argument Retrieval.