Collective motion in driven or self-propelled particle systems is a topic of recent interdisciplinary interest. Within physics, following the works of Vicsek et al. and Toner and Tu, most progress was achieved by studying microscopic point-like particles models and their continuous descriptions. For the simplest situation in which the surrounding fluid can be neglected ("dry flocking") and the sole interaction is some local effective alignment, a picture of basic universality classes has emerged.

When dealing with real systems, hard core repulsion comes into play. In a recent experiment of vibrated polar disks, i.e. millimeter-size objects with a built-in oriented axis, we have shown that hard core repulsion and self-propulsion produce some eff ective alignment leading at large scale to collective streams and anomalous, "giant" number fluctuations. More recently, it was shown that a slowing down of the self propulsion velocity with the local density could lead to a sharp clustering transition. Hard core repulsive systems will a priori experience such an eff ect because of self-trapping, glassy like phenomenology. Hence core repulsion and self propulsion build up both effective alignment and effective slowing down. Each of these eff ect produce very different transitions.

In the present talk I will discuss the interplay between these two transitions in the light of both
experimental results obtained in various systems such as walking grains, swimming droplets, rolling colloids and numerical results for self propelled hard discs and for point-like particles within a model encompassing both effects.