  The rapid advancements in quantum computing necessitate a scientific and
rigorous approach to the construction of a corresponding software ecosystem, a
topic underexplored and primed for systematic investigation. This chapter takes
an important step in this direction: It presents scientific considerations
essential for building a quantum software ecosystem that makes quantum
computing available for scientific and industrial problem solving. Central to
this discourse is the concept of hardware-software co-design, which fosters a
bidirectional feedback loop from the application layer at the top of the
software stack down to the hardware. This approach begins with compilers and
low-level software that are specifically designed to align with the unique
specifications and constraints of the quantum processor, proceeds with
algorithms developed with a clear understanding of underlying hardware and
computational model features, and extends to applications that effectively
leverage the capabilities to achieve a quantum advantage. We analyze the
ecosystem from two critical perspectives: the conceptual view, focusing on
theoretical foundations, and the technical infrastructure, addressing practical
implementations around real quantum devices necessary for a functional
ecosystem. This approach ensures that the focus is towards promising
applications with optimized algorithm-circuit synergy, while ensuring a
user-friendly design, an effective data management and an overall
orchestration. Our chapter thus offers a guide to the essential concepts and
practical strategies necessary for developing a scientifically grounded quantum
software ecosystem.
