At frequencies of approximately 100 GHz and above, rectangular metallic waveguide is often the preferred transmission medium for making reliable, precision, measurements. A prerequisite for these types of measurements is the use of the correct types of waveguide interface, and, the need to follow the correct operating procedures. This document is aimed at people interested in making reproducible measurements with relatively low measurement uncertainty, in rectangular waveguide, at these frequencies. It can be considered as a supporting document for existing standards that are available for waveguide and waveguide interfaces used at these frequencies (e.g. as published by IEEE, IEC, MIL, EIA, etc). This document concentrates on rectangular waveguides. However, many aspects that are presented might also be applicable to circular waveguides.