The Swampland program aims to distinguish effective theories that can be completed into quantum gravity in the ultraviolet from those that cannot. This article provides an introduction to the field, assuming only knowledge of quantum field theory and general relativity. It also offers a comprehensive review of the range of ideas in the field, from the Weak Gravity Conjecture to the de Sitter conjecture. The Swampland is defined as the set of (apparently) consistent effective field theories that cannot be completed into quantum gravity in the ultraviolet. String theory may lead to a large Landscape of effective low-energy theories, but there is an even larger Swampland of effective theories that cannot come from string theory. The Swampland program uses three approaches: string theory constructions, quantum gravity arguments, and microscopic physics, to establish evidence for constraints. These approaches appear remarkably in sync, leading to results consistent with each other. Swampland criteria typically have a universal structure. A class of effective QFTs self-consistent up to a cutoff scale Λ_QFT is coupled to gravity, leading to a new energy scale Λ_Swamp above which the theory becomes inconsistent. The scale Λ_Swamp is the point where the theory must start to 'plan ahead' to be consistent with quantum gravity in the ultraviolet. Depending on the parameters, Λ_Swamp may be above or below Λ_QFT, affecting the effective theory. The Swampland program is guided by string theory, which provides evidence for conjectures through known vacua. The criteria for not being in the Swampland are formulated in terms of properties of the low-energy effective theory. The Swampland program aims to uncover underlying microscopic physics that leads to these constraints. The program includes the Weak Gravity Conjecture, the Swampland Distance Conjecture, and the de Sitter conjecture. These criteria are supported by various arguments, including quantum gravity arguments and string theory constructions. The program also explores the emergence of dynamics in the infrared and the implications for cosmology.The Swampland program aims to distinguish effective theories that can be completed into quantum gravity in the ultraviolet from those that cannot. This article provides an introduction to the field, assuming only knowledge of quantum field theory and general relativity. It also offers a comprehensive review of the range of ideas in the field, from the Weak Gravity Conjecture to the de Sitter conjecture. The Swampland is defined as the set of (apparently) consistent effective field theories that cannot be completed into quantum gravity in the ultraviolet. String theory may lead to a large Landscape of effective low-energy theories, but there is an even larger Swampland of effective theories that cannot come from string theory. The Swampland program uses three approaches: string theory constructions, quantum gravity arguments, and microscopic physics, to establish evidence for constraints. These approaches appear remarkably in sync, leading to results consistent with each other. Swampland criteria typically have a universal structure. A class of effective QFTs self-consistent up to a cutoff scale Λ_QFT is coupled to gravity, leading to a new energy scale Λ_Swamp above which the theory becomes inconsistent. The scale Λ_Swamp is the point where the theory must start to 'plan ahead' to be consistent with quantum gravity in the ultraviolet. Depending on the parameters, Λ_Swamp may be above or below Λ_QFT, affecting the effective theory. The Swampland program is guided by string theory, which provides evidence for conjectures through known vacua. The criteria for not being in the Swampland are formulated in terms of properties of the low-energy effective theory. The Swampland program aims to uncover underlying microscopic physics that leads to these constraints. The program includes the Weak Gravity Conjecture, the Swampland Distance Conjecture, and the de Sitter conjecture. These criteria are supported by various arguments, including quantum gravity arguments and string theory constructions. The program also explores the emergence of dynamics in the infrared and the implications for cosmology.