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The organization of eukaryotic genomes is characterized by the presence of distinct euchromatic and heterochromatic sub-nuclear compartments. In Saccharomyces cerevisiae heterochromatic loci, including telomeres and silent mating type loci, form clusters at the nuclear periphery. We have employed live cell 3-D imaging and chromosome conformation capture (3C) to determine the contribution of nuclear positioning and heterochromatic factors in mediating associations of the silent mating type loci. We identify specific long-range interactions between HML and HMR that are dependent upon silencing proteins Sir2p, Sir3p, and Sir4p as well as Sir1p and Esc2p, two proteins involved in establishment of silencing. Although clustering of these loci frequently occurs near the nuclear periphery, colocalization can occur equally at more internal positions and is not affected in strains deleted for membrane anchoring proteins yKu70p and Esc1p. In addition, appropriate nucleosome assembly plays a role, as deletion of ASF1 or combined disruption of the CAF-1 and HIR complexes abolishes the HML-HMR interaction. Further, silencer proteins are required for clustering, but complete loss of clustering in asf1 and esc2 mutants had only minor effects on silencing. Our results indicate that formation of heterochromatic clusters depends on correctly assembled heterochromatin at the silent loci and, in addition, identify an Asf1p-, Esc2p-, and Sir1p-dependent step in heterochromatin formation that is not essential for gene silencing but is required for long-range interactions., Author Summary Chromosomes are non-randomly positioned inside cells, and this organization is relevant for genome regulation. Spatial clustering of heterochromatic loci provides a striking example of nuclear compartmentalization. In S. cerevisiae, the presence of heterochromatic sub-nuclear domains has been well established, but their mechanisms of formation are not fully understood. Here, we analyzed the DNA elements and protein complexes that are critical for formation of heterochromatic clusters. We focused on heterochromatic regions on chromosome III—the two telomeres, as well as the silent mating type loci HML and HMR, located on the left and right end of the chromosome, respectively. We employed live cell 3-D imaging and chromosome conformation capture (3C) and found that these loci specifically interact most prominently near silencer elements that flank the loci. Analysis of a panel of mutants showed that complexes involved in silencing are also involved in long-range interactions. Interestingly, we find that heterochromatic interactions are mechanistically distinct from silencing and independent of tethering to the nuclear periphery. Our results indicate that formation of heterochromatic clusters depends on correctly assembled heterochromatin, and point to a step in heterochromatin formation that is not essential for gene silencing but is required for long-range interactions between heterochromatic loci.