This paper presents electric-magnetic duality in N=1 supersymmetric non-Abelian gauge theories in four dimensions. The author demonstrates that two different gauge theories, with different gauge groups and quark representations, can lead to the same non-trivial long-distance physics. The quarks and gluons of one theory can be interpreted as solitons (non-Abelian magnetic monopoles) of the elementary fields of the other theory. The weak coupling region of one theory maps to the strong coupling region of the other. When one theory is Higgsed by a squark expectation value, the other becomes confined. Massless glueballs, baryons, and Abelian magnetic monopoles in the confining description correspond to weakly coupled quarks in the dual Higgs description. The paper discusses the non-Abelian Coulomb phase of supersymmetric QCD, where the light spectrum includes non-Abelian gluons and quarks. It shows how duality helps understand this phase. The theory is based on an SU(Nc) gauge theory with Nf flavors of quarks. The interesting gauge invariant operators are M, B, and B̃. For Nf ≥ Nc, the quantum theory has a moduli space of inequivalent vacua. For Nf = Nc, this space is different from the classical one. For Nf = Nc + 1, the two spaces are identical but the interpretation of the singularity at the origin is different. The paper argues that for 3Nc/2 < Nf < 3Nc, the theory at the origin of the moduli space is an interacting conformal field theory of quarks and gluons. This theory has two dual descriptions: the original "electric" variables and the dual "magnetic" variables. The electric description is an SU(Nc) gauge theory with Nf flavors, while the dual theory is an SU(Nf - Nc) theory with Nf flavors and an additional gauge invariant massless field. The quarks and gluons of one description can be thought of as solitons (magnetic monopoles) of the quarks and gluons of the dual theory. The paper also discusses the duality in SO(Nc) theories with Nf quarks in the vector representation. The dual of this theory is an SO(Nf - Nc + 4) gauge theory with Nf flavors and an additional gauge invariant massless field. The paper concludes that the theory and its dual have the same global symmetries. The gauge group of the dual theory is generally different from the original one. The duality makes sense in interacting scale-invariant theories, which do not have a well-defined particle interpretation and can be described by different sets of massless interacting particles. The paper also discusses the implications of duality for the behavior of gauge theories, confinement, and the Higgs mechanism.This paper presents electric-magnetic duality in N=1 supersymmetric non-Abelian gauge theories in four dimensions. The author demonstrates that two different gauge theories, with different gauge groups and quark representations, can lead to the same non-trivial long-distance physics. The quarks and gluons of one theory can be interpreted as solitons (non-Abelian magnetic monopoles) of the elementary fields of the other theory. The weak coupling region of one theory maps to the strong coupling region of the other. When one theory is Higgsed by a squark expectation value, the other becomes confined. Massless glueballs, baryons, and Abelian magnetic monopoles in the confining description correspond to weakly coupled quarks in the dual Higgs description. The paper discusses the non-Abelian Coulomb phase of supersymmetric QCD, where the light spectrum includes non-Abelian gluons and quarks. It shows how duality helps understand this phase. The theory is based on an SU(Nc) gauge theory with Nf flavors of quarks. The interesting gauge invariant operators are M, B, and B̃. For Nf ≥ Nc, the quantum theory has a moduli space of inequivalent vacua. For Nf = Nc, this space is different from the classical one. For Nf = Nc + 1, the two spaces are identical but the interpretation of the singularity at the origin is different. The paper argues that for 3Nc/2 < Nf < 3Nc, the theory at the origin of the moduli space is an interacting conformal field theory of quarks and gluons. This theory has two dual descriptions: the original "electric" variables and the dual "magnetic" variables. The electric description is an SU(Nc) gauge theory with Nf flavors, while the dual theory is an SU(Nf - Nc) theory with Nf flavors and an additional gauge invariant massless field. The quarks and gluons of one description can be thought of as solitons (magnetic monopoles) of the quarks and gluons of the dual theory. The paper also discusses the duality in SO(Nc) theories with Nf quarks in the vector representation. The dual of this theory is an SO(Nf - Nc + 4) gauge theory with Nf flavors and an additional gauge invariant massless field. The paper concludes that the theory and its dual have the same global symmetries. The gauge group of the dual theory is generally different from the original one. The duality makes sense in interacting scale-invariant theories, which do not have a well-defined particle interpretation and can be described by different sets of massless interacting particles. The paper also discusses the implications of duality for the behavior of gauge theories, confinement, and the Higgs mechanism.