About 50 years ago, Wilhelm Wiederhold wrote “Offene Fragen der Filtertechnik” (Open questions on filter technology) in the journal Gas und Wasserfach. His questions centred on 5 topics: the Boucher microstraining index and its application to sand filtration; the Michau pressure diagrams through the depth of a filter; the design and use of experimental filter columns; a multiple layer filter model comprising perforated bronze strainers; fluid flow regimes in porous media; backwashing with air scour followed by water fluidisation of a sand bed. This is reviewed in the light of the advances in the understanding of filter behaviour which have accrued over the past 50 years. Both theory and experimental techniques have enabled some of Wiederhold's questions to be answered, and for some of his ideas to be corrected. Nevertheless, our knowledge of filtration has raised more questions than have been answered. Wiederhold's ideas were a springboard for advances to be made, particularly experimentally. But new challenges face filtration technology, not least that granular bed filtration may disappear in favour of the progress of membrane technology.