Original Title: What To Do In Case Of What to do in the case of...? is a thoracic transplant-oriented Guidebook for the initial evaluation and diagnosis of common syndromes in cardiothoracic transplant recipients. This manual has been written by a wide international representation of authors with different areas of expertise, to give a multifaceted response to a broad range of clinical syndromes encountered after cardiothoracic transplantation. This project has been designed as an educational and practical tool for physicians, involved or not in the transplant field, to improve the management of transplant recipients. Indeed, delays in accurate diagnosis can lead to increased morbidity and mortality in this population. The manual has been structured in 11 sections, each divided into different chapters dealing with a general topic. Each chapter includes the most frequent etiologies of a clinical syndrome or finding, together with different diagnostic tests according to their priority in a transplant recipient. Every chapter has been written by two authors, at least one being an expert in the field of transplantation or infectious diseases. In addition, as a new diagnostic educational tool, each chapter has been reviewed by four different experts in the field of transplant. This project has been possible thanks to the creativity, wisdom, experience, and altruism of more than 150 professionals. I would like to thank my co-editor Fernanda Silveira and Lara Danziger-Isakov, leaders in the Infectious Diseases area, for their constant support during a period of more than five years needed to fully develop this ambitious project. Both editors want to express our gratitude to the authors for their excellent contributions, and also to the associate editors, standard and guideline reviewers and Maryl Johnson for assisting us during the long review process to improve the quality of the final manual. Finally, our heartfelt acknowledgment to ISHLT staff (Susie Newton, Megan Barrett and Amanda Rowe) for their excellent job organizing this complex project and James Kirklin for his positive judgment and attitude in publishing it. We hope this guidebook, with suggestions for the first steps in evaluation and differential diagnosis of common clinical situations, will help to improve the quality of management of our patients and thus avoid unnecessary diagnostic tests and delays in providing appropriate treatment.