The process of planet formation is dened by a multiplicity of physical and chemical effects. Observations show that { starting from sub-micrometer sizes { particles have to reach planetesimal sizes in approximately three million years. This implies growth over at least thirty decades in mass. Within this work, two aspects are treated in detail: the collisional dynamics of dust particles and their motion in protoplanetary disks. The material properties and their in uence on the collisional behaviour as well as the eect of reaccretion of ejecta in small impactor - large target collisions are investigated. It is shown that both aspects can have signicant impact on the growth process in certain areas of the disk. The motion of particles in protoplanetary disks is caused by dierent forces. A new analytic formula for photophoretic forces aecting irradiated particles in a gaseous environment with temperatures deviating from the surrounding gas is presented. Applying this formula to particles in pre-transitional disks results in a self-sustained recycling mechanism which is established within the inner parts of those disks. Hence, this process can explain the stability of the inner dust disk while no material is replenished by the outer disk through the gap.