Kleisli categories, T-categories and internal categories

We investigate the properties of the Kleisli category KlT of a monad (T,λ,μ) on a category E and in particular the existence of (some kind of) pullbacks. This culminates when the monad is cartesian. In this case, we show that any T-category in E in the sense of A. Burroni coincides with a special kind of internal category in KlT . So, it is the case in particular for T -operads and T -multicategories. More unexpectedly, this, in turn, sheds new lights on internal categories and n-categories.