In this thesis we study algebraic structures in M-theory, in particular the exceptional Lie algebras arising in dimensional reduction of its low energy limit, eleven-dimensional supergravity. We focus on e8 and its infinite-dimensional extensions e9 and e10. We review the dynamical equivalence, up to truncations on both sides, between eleven-dimensional supergravity and a geodesic sigma model based on the coset E10/K(E10), where K(E10) is the maximal compact subgroup. The description of e10 as a graded Lie algebra is crucial for this equivalence. We study generalized Jordan triple systems, which are closely related to graded Lie algebras, and which may also play a role in the description of M2-branes using three-dimensional superconformal theories. The introductory part is followed by five research papers. In Paper I we show that the spinor and vector-spinor representations of k(e10) in the fermionic extension of the original E10 coset model lead, upon restriction to k(e9), to the R-symmetry transformations in eleven-dimensional supergravity reduced to two dimensions. Paper II provides an explicit expression for the primitive E8 invariant tensor with eight symmetric indices, which is expected to appear in M-theory corrections in the reduction to three dimensions. In Paper III we show that e8, e9 and e10 can be constructed in a unified way from a Jordan algebra, via generalized Jordan triple systems. Also Paper IV deals with generalized Jordan triple systems, but in the context of superconformal M2-branes. We show that the recently proposed theories with six or eight supersymmetries can be expressed in terms of a graded Lie algebra. In Paper V we return to the bosonic E10 coset model, and apply it to gauged maximal supergravity in three dimensions.