The objective of this paper is decentralized swarm aggregation with collision avoidance. Specifically, each agent is assigned with a control law which is the sum of two elements: a repulsive potential field, which is responsible for the collision avoidance objective, and an attractive potential field, that forces the agents to converge to a configuration where they are close to each other. It is shown that under the proposed control law agents converge to a configuration where each agent is located at a bounded distance from each of its neighbors. Connectivity of the communication graph is then shown to be a sufficient condition for swarm aggregation. When a global objective is imposed, namely aggregation of the robotic swarm close to a desired location we show that only one agent, which plays the role of the leader of the swarm, has to be aware of this objective. The leader is shown to be able to drag the swarm around the desired location. We also show that these results are applied to both cases of robots satisfying single integrator kinematics and kinematic unicycle-type robots.