Biofilms　can　retain　microorganisms　with　very　different　growth　kinetics　and　different　electron　acceptor　preferences,　due　to　their　natural　redox　zonation.　Denitrifying　biofilm　processes　are　crucial　biological　processes　for　the　treatment　of　nitrogen-polluted　water.　Heterotrophic,　autotrophic,　and　methane-supported　denitrifying　biofilms　have　been　widely　utilized　for　removing　nitrogenous　contaminants　in　recent　years.　Heterotrophic　denitrification　is　a　conventional　approach　to　conduct　a　respiratory　process　with　nitrate　or　nitrite　as　the　terminal　electron　acceptor　and　organic　carbon　serving　as　the　main　electron　donor.　Autotrophic　denitrification　is　a　cost-effective　process　for　nitrogen　removal　with　reduced　inorganic　compounds　acting　as　the　electron　donor,　such　as　hydrogen　and　sulfur.　Methane　could　also　be　used　directly　as　an　electron　donor　to　drive　denitrification　in　the　methane-based　denitrification　process.　While　the　controlled　biofilm-based　systems　are　capable　of　enhancing　the　nitrogen　removal　efficiency　by　these　denitrifiers,　this　review　aimed　to　provide　a　comprehensive　and　up-to-date　summary　of　these　important　denitrifying　biofilm　processes　in　attached-growth　reactors　from　the　perspectives　of　microbiology,　reactor　configurations,　controlling　factors,　nitrous　oxide　emissions,　and　coexistence　with　other　microorganisms.　The　technical　challenges　and　associated　strategies　towards　improving　the　performance　of　each　denitrifying　biofilm　process　are　then　discussed.　The　outlook　for　the　larger　engineering　scalability　of　these　processes　is　also　put　forward　to　facilitate　their　applications　for　higher　treatment　capacities.　©　The　Royal　Society　of　Chemistry.