As frequency increases the wavelength gets smaller and therefore the size of waveguides shrink. At terahertz frequencies the size of a waveguide is such that it is difficult to fabricate using conventional milling, especially when fine structures are required when making filters. Here we present an overview of work on micromachined circuits for these very high frequencies, initially demonstrating waveguides can be constructed with a loss as good as normal metals and then describing the design and measurements of some waveguide filters.