Optical measurements are essential for many microfluidic devices, offering effective, quick, and reliable analysis methods on small sample volumes, for instance on a droplet of blood. In particular, microscopy is a powerful tool for monitoring and analysing cellular behaviour or fluid properties. Furthermore, microscopy is a flexible technique that can also be combined to manipulate cells or particles inside microfluidic channels using laser beams. This document is written to serve as a guideline for researchers and engineers working in the field of microfluidics with optical interfaces. This document aims to provide direction to microfluidic engineers and scientists about what to consider when designing the device to ensure proper function of the optical parts. As this document is created with the help of experts working on a diverse range of microfluidic products, it is expected to be applicable to a wide range of microfluidic products. The diversity in microfluidics applications and technologies is too large to create a comprehensive guideline; this document must be seen as a first step in the design process.