Classic psychedelics such as psilocybin, LSD, ayahuasca, 5-MeO-DMT, and mescaline are multifaceted substances that have been used for healing and ritual purposes for millennia. In recent years, scientific interest in their therapeutic potential has resurged, yielding promising results for the treatment of mental health conditions, including depression, anxiety, and substance use disorders. At the same time, growing public interest and enthusiastic media coverage have contributed to increasing use in non-clinical contexts. Although psychedelics are considered not addictive and exhibit very low toxicity, their use can, in some cases, be associated with considerable risks and complications. To date, these risks remain underresearched, and many mental health professionals are insufficiently informed about their pathophysiology, differential diagnosis and clinical management. This book seeks to address this gap. From an interdisciplinary perspective, it aims to contribute to a more nuanced and balanced understanding of the benefit–risk ratio and to offer practical guidance for managing complications that may arise when psychedelics are used in therapy and beyond.  The chapter "Classifying Psychedelic-Related Complications" is available open access under the Creative Commons Attribution Non Commercial - No Derivatives 4.0 International License via link.springer.com.