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.

This book constitutes the refereed proceedings of the 12th International Conference of the CLEF Association, CLEF 2021, held virtually in September 2021. The conference has a clear focus on experimental information retrieval with special attention to the challenges of multimodality, multilinguality, and interactive search ranging from unstructured to semi structures and structured data. The 11 full papers presented in this volume were carefully reviewed and selected from 21 submissions. This year, the contributions addressed the following challenges: application of neural methods for entity recognition as well as misinformation detection in the health area, skills extraction in job-match databases, stock market prediction using financial news, and extraction of audio features for podcast retrieval. In addition to this, the volume presents 5 "best of the labs" papers which were reviewed as full paper submissions with the same review criteria. 12 lab overview papers were accepted and represent scientific challenges based on new data sets and real world problems in multimodal and multilingual information access.