Heterogeneous polymerization techniques are industrially relevant, scalable methods to synthesize polymer colloids and they have emerged in recent decades as viable methods to produce biphasic and morphologically complex polymer particles. The overarching goal of this thesis is to understand how heterogeneous polymerization techniques can be modified and optimized to control particle morphology through the systematic study of process parameters. Chapter 1 introduces relevant nomenclature and provides a brief historical perspective on the development of heterogeneous polymerization. This chapter also reviews the state-of-the-art techniques used to prepare morphologically complex polymer particles, with an emphasis on the heterogeneous polymerization processes studied in following chapters. In Chapter 2, the one-step synthesis of biphasic, cavity-forming particles is demonstrated, and the systematic study of process parameters is used to elucidate a new heterogeneous nucleation mechanism in dispersion copolymerization. Biphasic particles are produced by exploiting the insolubility of the zwitterionic monomer, sulfobetaine methacrylate (SBMA), in mixtures of water and isopropanol. SBMA-rich colloidal aggregates act as nucleation sites for biphasic particle formation, ultimately being encapsulated in a styrene-rich particle shell. The hydrophilicity of SBMA promotes expulsion of the core phase simply by washing the biphasic particles in water. Chapters 3 and 4 leverage the competition between thermodynamic and kinetic factors controlling polymerization-induced phase separation (PIPS) in seeded emulsion polymerization (SEP) to modulate particle morphology and size. In ix Chapter 3, using an established SEP protocol, monomer feed composition and concentration are studied as parameters to control the viscosity within the polymerizing seeded emulsion microreactors. The mechanism of PIPS is found to differ significantly as a function of monomer feed composition, while monomer feed concentration can be modulated to create a suite of patchy particle morphologies. Sacrificial dissolution of one polymer phase of the biphasic particles formed during SEP yields new porous and bowl-shaped structures. The study of SEP is expanded in Chapter 4 with the goal of developing a universal framework describing both particle morphology and size control. Particle size, reaction temperature, and initiator concentration are systematically investigated, and results reinforce the significant dependence on viscosity within the polymerizing seeded emulsions as a key parameter controlling morphology development. Finally, Chapter 5 summarizes the conclusions of the work in the previous chapters and suggests future research directions.