  We study the (standard) cohomology $H^\bullet_{st}(E)$ of a Courant algebroid
$E$. We prove that if $E$ is transitive, the standard cohomology coincides with
the naive cohomology $H_{naive}^\bullet(E)$ as conjectured by Stienon and Xu.
For a general Courant algebroid we define a spectral sequence converging to its
standard cohomology. If $E$ is with split base, we prove that there exists a
natural transgression homomorphism $T_3$ (with image in $H^3_{naive}(E)$)
which, together with the naive cohomology, gives all $H^\bullet_{st}(E)$. For
generalized exact Courant algebroids, we give an explicit formula for $T_3$
depending only on the \v{S}evera characteristic clas of $E$.
