We present a thorough description of high-performance thin-film optical filters with high flatness. These components can combine several tens or hundreds of layers and are manufactured using plasma-assisted reactive magnetron sputtering. Stress compensation is achieved using dual-side coatings with appropriate spectral function. Examples are described of highly reflecting mirrors at 515 nm with 15 nm flatness peak-to-valley up to and over 75 mm diameter aperture, narrow bandpass filters, and filters with broadband controlled transmission.