By his own account, in his *Notice sur les travaux scientifiques*, the main theme of his works (numbering 186 and published throughout the period 1893-1947) was the theory of Lie groups. He began by working over the foundational material on the complex simple Lie algebras, tidying up the previous work by Engel and Killing. This proved definitive, as far as the classification went, with the identification of the four main families and the five exceptional cases. He also introduced the algebraic group concept, which was not to be developed seriously before 1950.

He defined the general notion of anti-symmetric differential form, in the style now used; his approach to Lie groups through the Maurer-Cartan equations required 2-forms for their statement. At that time what were called Pfaffian systems (i.e. first-order differential equations given as 1-forms) were in general use; by the introduction of fresh variables for derivatives, and extra forms, they allowed for the formulation of quite general PDE systems. Cartan added the exterior derivative, as an entirely geometric and coordinate-independent operation. It naturally leads to the need to discuss *p*-forms, of general degree *p*. Cartan writes of the influence on him of Riquier’s general PDE theory.

With these basics – Lie groups and differential forms – he went on to produce a very large body of work, and also some general techniques such as moving frames, that were gradually incorporated into the mathematical mainstream.

In the *Travaux*, he breaks down his work into 15 areas. Using modern terminology, they are these:

- Lie groups
- Representations of Lie groups
- Hypercomplex numbers, division algebras
- Systems of PDEs, Cartan-Kähler theorem
- Theory of equivalence
- Integrable systems, theory of prolongation and systems in involution
- Infinite-dimensional groups and pseudogroups
- Differential geometry and moving frames
- Generalised spaces with structure groups and connections, Cartan connection, holonomy, Weyl tensor
- Geometry and topology of Lie groups
- Riemannian geometry
- Symmetric spaces
- Topology of compact groups and their homogeneous spaces
- Integral invariants and classical mechanics
- Relativity, spinors

To look at some of those less mainstream areas:

- the PDE theory has to take into account singular solutions (i.e. envelopes), such as are seen in Clairaut’s equation;
- the prolongation method is supposed to terminate in a system
*in involution*(this is an analytic theory, rather than smooth, and leads to the theory of formal integrability and Spencer cohomology); - the equivalence problem, as he put it, is to construct differential isomorphisms of structures (and discover thereby the invariants) by forcing their graphs to be integral manifolds of a differential system;
- the moving frames method, as well as being connected to principal bundles and their connections, should also use frames adapted to geometry;
- these days, the jet bundle method of Ehresmann is applied to use contact as a systematic equivalence relation.