This review article provides a pedagogical introduction into various classes of chiral string compactifications to four dimensions with D-branes and fluxes. The main concern is to provide all necessary technical tools to explicitly construct four-dimensional orientifold vacua, with the final aim to come as close as possible to the supersymmetric Standard Model. Furthermore, we outline the available methods to derive the resulting four-dimensional effective action. Finally, we summarize recent attempts to address the string vacuum problem via the statistical approach to D-brane models. The article discusses D-branes, orientifolds, and fluxes in the context of string compactifications. It covers the basic concepts of D-branes, their effective field theory description, and their role in supersymmetric and non-supersymmetric models. It also discusses orientifolds, their properties, and their role in constructing four-dimensional vacua. The article then presents various models of string compactifications, including intersecting D6-brane models in type IIA orientifolds, magnetized D-brane models in type IIB orientifolds, and Gepner model orientifolds. It also discusses the low-energy effective action, flux compactifications, and the string vacuum problem. The article concludes with a discussion of the challenges and open questions in string model building, including the problem of moduli stabilization, the string vacuum problem, and the search for realistic string models. It also discusses the role of statistical methods in addressing the string vacuum problem and the importance of supersymmetry in string theory. The article provides a comprehensive overview of the current state of research in string model building and highlights the key concepts and techniques used in the field.This review article provides a pedagogical introduction into various classes of chiral string compactifications to four dimensions with D-branes and fluxes. The main concern is to provide all necessary technical tools to explicitly construct four-dimensional orientifold vacua, with the final aim to come as close as possible to the supersymmetric Standard Model. Furthermore, we outline the available methods to derive the resulting four-dimensional effective action. Finally, we summarize recent attempts to address the string vacuum problem via the statistical approach to D-brane models. The article discusses D-branes, orientifolds, and fluxes in the context of string compactifications. It covers the basic concepts of D-branes, their effective field theory description, and their role in supersymmetric and non-supersymmetric models. It also discusses orientifolds, their properties, and their role in constructing four-dimensional vacua. The article then presents various models of string compactifications, including intersecting D6-brane models in type IIA orientifolds, magnetized D-brane models in type IIB orientifolds, and Gepner model orientifolds. It also discusses the low-energy effective action, flux compactifications, and the string vacuum problem. The article concludes with a discussion of the challenges and open questions in string model building, including the problem of moduli stabilization, the string vacuum problem, and the search for realistic string models. It also discusses the role of statistical methods in addressing the string vacuum problem and the importance of supersymmetry in string theory. The article provides a comprehensive overview of the current state of research in string model building and highlights the key concepts and techniques used in the field.