This review article, authored by Austin Joyce, Bhuvnesh Jain, Justin Khoury, and Mark Trodden, explores the development of theoretical models beyond the standardΛCDM (Lambda Cold Dark Matter) cosmological model, focusing on modified gravity as a potential solution to the cosmological constant problem and the accelerating universe. The authors discuss the current state of the field, the guiding principles for rigorous and consistent modifications of the standard model, and the prospects for empirical tests. The review begins by reviewing recent attempts to modify Einstein gravity in the infrared regime, emphasizing the concept that additional degrees of freedom introduced by these modifications must "screen" themselves from local tests of gravity. These screening mechanisms are categorized into three broad classes: those active in regions of high Newtonian potential, those involving first derivatives of the field, and those involving second derivatives of the field. Examples of each class include chameleon and symmetron mechanisms, galileon and massive gravity theories, and the Vainshtein mechanism. The authors describe each theory as an effective theory and discuss the prospects for completing them in more fundamental theories. They also detail experimental tests of these theories, including laboratory and solar system tests, as well as astrophysical and cosmological tests. The review concludes with a discussion of future tests that will be sensitive to different signatures of new physics in the gravitational sector. The article is structured to clearly indicate the relevance of different sections to theorists, observers, and experimentalists, aiming to serve as a useful reference for both audiences and to help bridge the gap between them.This review article, authored by Austin Joyce, Bhuvnesh Jain, Justin Khoury, and Mark Trodden, explores the development of theoretical models beyond the standardΛCDM (Lambda Cold Dark Matter) cosmological model, focusing on modified gravity as a potential solution to the cosmological constant problem and the accelerating universe. The authors discuss the current state of the field, the guiding principles for rigorous and consistent modifications of the standard model, and the prospects for empirical tests. The review begins by reviewing recent attempts to modify Einstein gravity in the infrared regime, emphasizing the concept that additional degrees of freedom introduced by these modifications must "screen" themselves from local tests of gravity. These screening mechanisms are categorized into three broad classes: those active in regions of high Newtonian potential, those involving first derivatives of the field, and those involving second derivatives of the field. Examples of each class include chameleon and symmetron mechanisms, galileon and massive gravity theories, and the Vainshtein mechanism. The authors describe each theory as an effective theory and discuss the prospects for completing them in more fundamental theories. They also detail experimental tests of these theories, including laboratory and solar system tests, as well as astrophysical and cosmological tests. The review concludes with a discussion of future tests that will be sensitive to different signatures of new physics in the gravitational sector. The article is structured to clearly indicate the relevance of different sections to theorists, observers, and experimentalists, aiming to serve as a useful reference for both audiences and to help bridge the gap between them.