In ultrarelativistic nuclear collisions heavy quarks, charm and beauty,
are produced almost immediately after the collision between the two colliding objects takes place. In fact, their production (proper) time can be estimated as $\tau_\mathrm{form}\approx 1/2m$ where $m$ is the mass of the heavy quarks: this gives $\tau_\mathrm{form}\approx 0.06$ fm/c for charm and $\tau_\mathrm{form}\approx 0.02$ fm/c for beauty. Consequently, charm and beauty can probe the entire evolution of the system produced by the collision, including the very early stage in which the strong, coherent gluon fields, called the Glasma, dominate the dynamics.
The early stage is then followed by the quark-gluon plasma stage, and finally by hadronization.
We briefly review the recent developments on the diffusion and the energy loss of charm and
beauty in ultrarelativistic heavy ion collisions, with particular emphasis on those in the early stage and on the estimate of the spatial diffusion coefficient of heavy quarks in the quark-gluon plasma.