In addition to critical Casimir forces between homogeneous surfaces, we experimentally study the interaction between colloidal particles and chemically patterned substrates immersed in a binary critical mixture. Chemical patterns are created by means of a focused ion beam or micro-contact-printing. Close to the critical point of the mixture, the particles are subjected to critical Casimir interactions with force components normal and parallel to the surface. Because the strength and sign of these interactions can be tuned by variations in the surface properties and the mixture´s temperature, critical Casimir forces allow the formation of highly ordered monolayers. In contrast to other aggregation mechanisms, critical Casimir forces are fully reversible which allows to thermally anneal the obtained structures. This will result in a largely reduced defect density being important for technical applications such as photonic or phononic crystals. Since critical Casimir forces are not restricted to micron-sized particles, the principle should be also applicable on smaller length scales.