Kurt Anderson
The Francis Crick Institute, London, United Kingdom

Tanmay A.M. Bharat
Sir William Dunn School of Pathology, University of Oxford, United Kingdom

Jose L. Carrascosa
Department of Macromolecular Structures, Centro Nacional de Biotecnologia (CNB‐CSIC), Madrid, Spain

Francisco Javier Chichón
Department of Macromolecular Structures, Centro Nacional de Biotecnologia (CNB‐CSIC), Madrid, Spain

Georg Fantner
Laboratory for Bio‐ and Nano‐instrumentation, School of Engineering, Interfaculty Institute of Bioengineering, Lausanne, Switzerland

Julia Fernandez‐Rodriguez
Centre for Cellular Imaging at Sahlgrenska Academy, University of Gothenburg, Sweden

Christopher J. Guérin
VIB Bioimaging Core, Ghent, VIB Inflammation Research Center, Ghent and Department of Molecular Biomedical Research, University of Ghent, Belgium

J. P. Hoogenboom
Imaging Physics, Delft University of Technology, The Netherlands

Eija Jokitalo
Helsinki Institute of Life Science, Institute of Biotechnology, University of Helsinki, Finland

Niels de Jonge
INM – Leibniz Institute for New Materials and Department of Physics, Saarland University, Saarbrücken, Germany

Judith Klumperman
Section Cell Biology, Center for Molecular Medicine, University Medical Center Utrecht, The Netherlands

Irina Kolotuev
University of Lausanne, EM Facility, Switzerland

R. I. Koning
Cell and Chemical Biology, Leiden University Medical Center, The Netherlands

A. J. Koster
Cell and Chemical Biology, Leiden University Medical Center, The Netherlands

Wanda Kukulski
Cell Biology Division, MRC Laboratory of Molecular Biology, Francis Crick Avenue, Cambridge, United Kingdom

Frank Lafont
Cellular Microbiology and Physics of Infection Group
Center for Infection and Immunity of Lille, CNRS UMR8204 – Inserm U1019 – Lille Regional University Hospital Center – Institut Pasteur de Lille – Univ. Lille, France

R. I. Lane
Imaging Physics, Delft University of Technology, The Netherlands

Saskia Lippens
BioImaging Core, VIB, Ghent, Belgium

Nalan Liv
Section Cell Biology, Center for Molecular Medicine, University Medical Center Utrecht, The Netherlands

Kristina D. Micheva
Stanford University School of Medicine, California, United States

Tommy Nilsson
The Research Institute of the McGill University Health Centre and McGill University, Montreal, Canada

Ardan Patwardhan
European Molecular Biology Laboratory, European Bioinformatics Institute (EMBL‐EBI), Wellcome Genome Campus, Hinxton, United Kingdom

Perrine Paul‐Gilloteaux
Structure Fédérative de Recherche François Bonamy, CNRS, INSERM, Université de Nantes, France

Christopher J. Peddie
Electron Microscopy Science Technology Platform, The Francis Crick Institute, London, United Kingdom

Eva Pereiro
Mistral beamline, ALBA Light Source, Cerdanyola del Vallès, Barcelona, Spain

A. Srinivasa Raja
Imaging Physics, Delft University of Technology, The Netherlands

Nicole L. Schieber
Cell Biology and Biophysics Unit, European Molecular Biology Laboratory, Heidelberg, Germany

Martin Schorb
European Molecular Biology Lab (EMBL), Heidelberg, Germany

Jason R. Swedlow
Centre for Gene Regulation and Expression, University of Dundee, United Kingdom

Correlative microscopy (CM), or more broadly correlative imaging (CI), aims to analyze a single sample by two or more distinct imaging modalities. By doing so, one should be able to extract more scientific insight than would have otherwise been possible using each imaging modality as a standalone technique. We have thus coined the expression 1 + 1 = 3 to explain the principle of CI. It should be noted that CI is NOT the process of imaging biological replicates with a variety of imaging techniques, which would be more properly referred to as comparative imaging.

Over the last two decades, the field of correlative imaging has seen a massive expansion in development and application, primarily driven by the need to link structure and function in a biological context. This expansion was facilitated by a number of factors, including the development of superresolution light microscopy, the resolution revolution in cryo‐electron microscopy (EM), and the volume revolution in scanning electron microscopy (SEM).

The correlative revolution began with the development of correlative light electron microscopy (CLEM), with an initial swell at the end of the 1980s that then exploded in terms of developments and publications in the early 2000s (see also Chapter 2). CLEM specifically combines a light and an electron microscopy modality to image the same sample, and is the best‐established CI methodology. In the early days, separate CLEM sessions in microscopy conferences would highlight technical advances in the field, and those, expanded into CI sessions, are now a mainstay at most microscopy conferences. As CI has matured, the most established workflows have shifted into the applications domain, and are often incorporated into mainstream scientific sessions at biological and, increasingly, physical sciences meetings. This important transformation shows that CI technology is now considered an established technique that can be applied to a wide variety of research questions.

Not all research questions will need a CI approach, but where the region of interest within the sample to be imaged is rare in space and/or time, CI can deliver “the needle in the haystack,” alongside significant savings in both time and resources. In addition, many scientific questions will require adaptation or optimization of an existing correlative workflow, or even development of a new CI approach. To this end, we have already collected a large number of CLEM approaches and published them in dedicated volumes of the Methods in Cell Biology book series (Volumes 111, 124, and 140). Here, we asked the authors of the chapters to describe their technical approach, highlight tips and tricks, and, importantly, to explain why they had chosen their approach to answer their biological research question. With continuing fast‐paced developments in the field, we have already received a number of queries for a fourth edition, for which we are compiling a list of chapters, with no shortage of new material available.

The feedback we have received on those books has been very positive. They are a resource that captures a snapshot of the state of the art in the field at the point of publication, and technology developers have used them as inspiration for the next iterations of new CI approaches. Having compiled the current state of the art, we were interested to look at where we are heading next. We asked leaders in different areas of CI to write down their thoughts on current limitations and how these could be solved, and what the next transformative technologies might be. Thus, this book is a snapshot of the current state of the art, but with additional musings and best‐guesses of leaders in the field as to what future generations of CI technology may look like.

We asked these experts an additional question, “What CI technology would you ideally use to answer our scientific questions?” This “blue‐skies daydreaming” exercise was not to be limited by the practicalities of current hardware and software solutions, and turned out to be fruitful in generating a call from the community for concerted efforts in specific areas, as well as delivering fascinating ideas that will undoubtedly drive new breakthroughs.

We fully realize that the chapters in this book are a personalized choice of topics and we may well have missed some of the next transformative CI technologies. We also recognize that some of the chapters will have a more general impact and will be valid for other research fields as well. We look forward to reading the book in 5 years’ and 10 years’ time, to see how the future of CI matches up to the expert predictions of 2017–19, when the book was written.

We hope you enjoy the read and that this book may be an inspiration for your own research.