  We study the loop clusters induced by Poissonian ensembles of Markov loops on
a finite or countable graph (Markov loops can be viewed as excursions of Markov
chains with a random starting point, up to re-rooting). Poissonian ensembles
are seen as a Poisson point process of loops indexed by 'time'. The evolution
in time of the loop clusters defines a coalescent process on the vertices of
the graph. After a description of some general properties of the coalescent
process, we address several aspects of the loop clusters defined by a simple
random walk killed at a constant rate on three different graphs: the integer
number line $\mathbb{Z}$, the integer lattice $\mathbb{Z}^d$ with $d\geq 2$ and
the complete graph. These examples show the relations between Poissonian
ensembles of Markov loops and other models: renewal process, percolation and
random graphs.
