Abstract: Spin qubits in Si/SiGe heterostructures have several advantages as scalable qubit platforms, including their small size, their long coherence times, and their reliance upon conventional semiconductor fabrication methods. However, microscopic disorder in the semiconductor structure impact these qubits in a variety of ways, reducing qubit yield. In particular, the valley energy splitting (the energy gap between the two low-lying conduction band valley states) is widely variable, and highly sensitive to microscopic disorder. In this dissertation, we study the effects of disorder on spin qubits formed from quantum dots in Si/SiGe heterostructures, focusing particularly on the valley energy splitting. We demonstrate that alloy disorder (disorder due to the random arrangement of Si and Ge atoms in the SiGe alloy) has a profound impact on these qubits. We develop a theory to explain the impact of alloy disorder on the valley splitting, and we compare the results of this theory to a variety of experiments, finding good quantitative agreement. We demonstrate that alloy disorder determines the valley splitting in most realistic devices, and we propose a high-Ge heterostructure that enhances alloy disorder in order to increase average valley splittings. We also examine the impact of alloy disorder on long-distance qubit connectivity via conveyor-mode electron shuttling. We demonstrate that alloy disorder leads to valley excitations, causing quantum information to leak out of the qubit subspace. We develop a variety of schemes to mitigate these excitations, by either avoiding valley excitations or mitigating their impact, providing recipes for high-fidelity spin shuttling in several device regimes.