Preface 6 variety in conceptualisations on what it is, how it is shown, what it addresses,and what it requires. The use and development of critical thinking are regularly and intentionally supported both in the labour market and higher education, despite diverse and clear indications that all that supporting of critical thinking, all that teaching of critical thinking are not always equally effective. Different conceptualisations of critical thinking result in different interpretations of the value of particular interventions, and, in turn, these different interpretations result in different reactions to those interventions. In addition to that insight, the monograph also extends an invitation for more research on fostering critical thinking, both by creating situations that afford critical thinking and by developing what is needed to engage in critical thinking. Here, too, we can only hope that the invitation will be widely accepted.The monograph highlights the importance of critical thinking, that is, why it is equally important to research it. It also highlights the complexity of critical thinking, stressing the need for research that acknowledges that complexity. The monograph recognises and illustrates that critical thinking deserves and requires multiple research approaches. Understanding critical thinking, the practice of critical thinking, the fostering of critical thinking, and the development of critical thinking requires knowledge and an in-depth understanding of the literature on critical thinking; it also requires engagement in empirical research, from qualitative and quantitative to descriptive, explanatory and interventionist. The monograph reveals that more conceptual and more empirical research need one another. Through instantiation, empirical research helps understand and validate conceptual research on critical thinking, while the use of conceptual literature is essential for deconstructing the findings in order to make them meaningful. The methodological richness and solid embeddedness in the (international) literature of this monograph might be intimidating, but extend an invitation to researchers and scholars in the domain of critical thinking to be especially critical when it comes to making methodological choices. Again, we can only hope that the invitation will be widely accepted.In The Society of the Spectacle, Guy Debord invites us to be critical and not to be 'spectators', writing: 'Spectators do not find what they want, they want what they find'. This monograph helps to not become spectators -to remain critical first and foremost about (the development of) critical thinking.