We present the design, development, and application of QHDL, a quantum hardware description language specifically designed for tightly-coupled quantum–classical computing systems. Together with the language design principles, we describe the QHDL compiler, debugger, and co-simulation infrastructure. We showcase the benefits of using a quantum–classical integrated approach in four use cases, requiring close quantum–classical device interaction: Bell's pair circuit, dynamic delay, Quantum Fourier Transform (QFT), and teleportation. To interface with QHDL, we propose to use synchronous techniques that are commonplace in digital hardware design. We illustrate examples of modeling both loosely-coupled and tightly-coupled quantum circuits that use so-called measurement-in-the-middle by utilizing these techniques in QHDL. For clock-cycle accurate implementations, we propose implementing such classical modules as programmable hardware blocks using Register-Transfer Level (RTL) or gate-level approaches. These approaches provide the highest coupling performance and are feasible to be implemented in state-of-the-art control systems.