Abstract: Fault-tolerant quantum computers will not simply run today’s quantum programs on more reliable hardware. Error correction imposes constraints that propagate across the stack, from hardware and classical control to compilation and program design. I will describe a layered framework for understanding how different error-correcting codes shape the design of fault-tolerant architectures. Comparing concrete surface-code and quantum-LDPC-code architectures, I will explain how quantum LDPC codes can substantially reduce physical-qubit requirements, but can also increase algorithm runtime due to constraints of the fault-tolerant logical instruction set. The resulting comparison points toward a broader theoretical programme: understanding how code structure, fault-tolerant logical operations, compilation, and hardware constraints jointly determine the resources required for scalable quantum computation.