The paper constructs and studies three-dimensional Chern-Simons-matter theories with gauge groups \( U(N) \times U(N) \) and \( SU(N) \times SU(N) \) that have explicit \( N = 6 \) superconformal symmetry. These theories are argued to describe the low-energy limit of \( N \) M2-branes probing a \( \mathbf{C}^4/\mathbf{Z}_k \) singularity, where \( k \) is the level of the Chern-Simons theory. For large \( N \), the theory is dual to M-theory on \( AdS_4 \times S^7/\mathbf{Z}_k \). The theory also has a 't Hooft limit, which is dual to type IIA string theory on \( AdS_4 \times \mathbf{CP}^3 \). For \( k = 1 \), the theory is conjectured to describe \( N \) M2-branes in flat space, although only six of the eight supersymmetries are explicitly realized. The paper provides evidence for this conjecture, similar to the evidence for mirror symmetry in three-dimensional gauge theories. When the gauge group is \( SU(2) \times SU(2) \), the theory has extra symmetries and becomes identical to the Bagger-Lambert theory. The paper discusses the moduli space of the theories, the spectrum of chiral operators, and Wilson lines, and explores the gravity duals of these theories.The paper constructs and studies three-dimensional Chern-Simons-matter theories with gauge groups \( U(N) \times U(N) \) and \( SU(N) \times SU(N) \) that have explicit \( N = 6 \) superconformal symmetry. These theories are argued to describe the low-energy limit of \( N \) M2-branes probing a \( \mathbf{C}^4/\mathbf{Z}_k \) singularity, where \( k \) is the level of the Chern-Simons theory. For large \( N \), the theory is dual to M-theory on \( AdS_4 \times S^7/\mathbf{Z}_k \). The theory also has a 't Hooft limit, which is dual to type IIA string theory on \( AdS_4 \times \mathbf{CP}^3 \). For \( k = 1 \), the theory is conjectured to describe \( N \) M2-branes in flat space, although only six of the eight supersymmetries are explicitly realized. The paper provides evidence for this conjecture, similar to the evidence for mirror symmetry in three-dimensional gauge theories. When the gauge group is \( SU(2) \times SU(2) \), the theory has extra symmetries and becomes identical to the Bagger-Lambert theory. The paper discusses the moduli space of the theories, the spectrum of chiral operators, and Wilson lines, and explores the gravity duals of these theories.