Researchers are familiar with the constraints and difficulties in researching and understanding complex populations. For example, children and expatriates can be defined as complex populations to collect data from because of one main feature that brings them together: they live between two worlds. It this book, I introduce a new methodology to conduct research with complex populations by using marketing as a meta-field and multi-method approaches to better communicate with children as well as to the understanding of management issues with expatriates. In this book, I present a compilation of findings and methods I have been using in researches aiming at understanding children as consumers as well as expatriates¿ adjustment process through their relationship with brands. These researches have been conducted with a longitudinal approach with participants from all continents for eight years. I learned from the results that using marketing as a meta-field associated with the step-by-step methodology enables to better interact with these populations and therefore to unearth deep feelings, emotions and concerns.