A converse to Lie's theorem for Leibniz algebras is found and generalized. The result is used to find cases in which the generalized property, called triangulable, is 2-recognizeable; that is, if all 2-generated subalgebras are triangulable, then the algebra is also. Triangulability joins solvability, supersolvability, strong solvability, and nilpotentcy as a 2-recognizeable property for classes of Leibniz algebras.Simultaneous triangulation of a set of linear transformations related to Leibniz algebras will be considered in this work. A necessary condition is that the minimum polynomials of the linear transformations are the products of linear factors. If the matrices commute, then this condition is sufficient. Lie's theorem is a generalization of this result, a result that is field dependent even in the algebraically closed case. Lie's theorem extends to Leibniz algebras. It can also be useful to have a condition on the set which guarantees simultaneous triangulation when extending to the algebraic closure, K, of the field of scalars, F . We will call this property, borrowing from Bowman, Towers and Varea [3], triangulable. Thus a Leibniz algebra A over a field F is triangulable on module M if the induced representation of K A on K M admits a basis such that the representing matrices are upper triangular. We find a condition on the Leibniz algebra that is equivalent to being triangulable. The condition is independent of the field and includes Lie's theorem. The condition is on A although triangulable refers to the action in the algebra that is over K. As an application, we consider to what extent an algebra is triangulable if all 2-generated subalgebras have the property. Such a property is called 2-recognizable, a concept investigated in groups, Lie algebras and Leibniz algebras for classes such as solvable, supersolvable, strongly solvable and, trivially, nilpotent and abelian. For an introduction to Leibniz algebras, see [1], [6] and [7].Let M be a module for the Leibniz algebra A. Let left and right multiplication by x ∈ A be denoted by T x and S x , respectively. If T x is nilpotent, then S x is also nilpotent and an Engle theorem holds; that is, A acts nilpotently on M [2]. Define A to be nil on M if this property holds. If I is an ideal of a Leibniz algebra A, then I is nil on A if and only if I is nilpotent. We will show that A is triangulable on itself if and only if A 2 is nilpotent. When A 2 is nilpotent, then A is called strongly solvable, a term introduced in [3].