I discuss the construction of L-infinity algebras including

the special case of the Courant algebroid. Then I elaborate on the

general relation between the data of an L-infinity algebra and a

classical field theory in theoretical physics. It is argued that there is a

one-to-one correspondence between consistent field theories and

L-infinity algebras (up to reasonably defined isomorphisms),

with the gauge algebra, interactions, etc. of a field theory being

encoded in the higher brackets of an L-infinity algebra.

I discuss double field theory as a non-trivial example.