The self-assembly of quantum dot (QD) islands in lattice-mismatched heteroepitaxy has been studied extensively. One of the objectives of recent research is the creation of long-range ordered arrays of QDs of uniform size, a major technological milestone that would pave the way for application of these nanostructures in electronic and optoelectronic devices. The formation of epitaxial QD islands is generally assumed to involve nucleation, a statistical process that would severely impede QD organization. Our recent observations by low-energy electron microscopy, however, have demonstrated that QD islands in heteroepitaxial systems such as SiGe/Si(100) can self-assemble in a coninuous process that avoids nucleation.@footnote 1@ With the observed nucleationless islanding, perfect periodic arrays of uniform QD islands may form spontaneously under suitable growth conditions or may be induced by weakly invasive growth modifiers that enhance existing ordering forces. We present a systematic study of the evolution of SiGe QD islands on Si(100) by combining growth with in-situ scanning tunneling microscopy. Our microscopic observations are analyzed to detect island interaction mechanisms that may act as driving forces for spontaneous spatial ordering in the nucleationless islanding process. First results on the use of growth modifiers to enhance or induce long-range order will be presented. @FootnoteText@ @footnote 1@ P. Sutter and M. G. Lagally; Phys. Rev. Lett., 84(20):4637-4640, 2000.