In this dissertation, we present in silico and in vitro work on the dynamics of cellular aggregation and rearrangement as cell-cell and cell-ECM interactions are systematically varied. Computationally, we explore the contributions of homotypic and heterotypic forces between cells and the ECM affect cellular self-assembly. We find that variation of homotypic and heterotypic forces generates both expected morphologies and previously unreported patterns. Among the newly discovered patterns are segmented states of alternating cell types, and an “onion” state, in which cells form multilayer-aggregates of two cell types. Experimentally we varied cell-ECM adhesive strength through selection of α5β1-integrin receptor expression in Chinese Hamster Ovary (CHO) Cells at two soluble fibronectin (sFn) concentrations. Second, to describe dual adhesive relations, we used a CHO cell line variants coexpressing integrin and N-cadherin surface receptors. We found previously unreported complex behaviors of aggregates in these experiments. For example, we found that at constant sFn concentration, aggregate cohesion grows linearly as α5β1 receptor density is increased from low to moderate levels. However, further increase in receptor expression causes an abrupt drop in tissue cohesion. We propose that the observed biphasic property of these aggregates may be due to depletion of sFn below a critical value in the aggregate microenvironment at high α5β1 expression level. We also found that a complicated interplay emerges when cell-ECM and cell-cell interactions mediate cellular aggregation and rearrangement. Thus, we describe two nodes of cellular interaction, cell-ECM and cell-cell/cell-ECM. For weak cell-ECM interactions, cells can still rearrange, and new cellular patterns (e.g. inverted structures) emerge. For high cell-ECM strengths, cells are bound to the matrix and cannot rearrange. For weak and high cell-ECM interactions, cell-cell governs final equilibrium configurations. We propose that these results have potential implications for embryonic development, for wound healing, and for cancer therapeutic applications.