<p>An understanding of the pinning of magnetic skyrmions to defects is crucial for the development of<br>
future spintronic applications. While pinning is desirable for a precise positioning of magnetic<br>
skyrmions it is detrimental when they are to be moved through a material.Weuse scanning tunneling<br>
microscopy (STM) to study the interaction between atomic scale defects and magnetic skyrmions that<br>
are only a few nanometers in diameter. The studied pinning centers range from single atom inlayer<br>
defects and adatoms to clusters adsorbed on the surface of our model system.Wefind very different<br>
pinning strengths and identify preferred positions of the skyrmion. The interaction between a cluster<br>
and a skyrmion can be sufficiently strong for the skyrmion to follow when the cluster is moved across<br>
the surface by lateral manipulation with the STMtip.</p