Calibration　is　a　process　of　comparing　model　results　with　field　data　and　making　the　appropriate　adjustments　so　that　both　results　agree.　Calibration　methods　can　involve　formal　optimization　methods　or　manual　methods　in　which　the　modeler　informally　examines　alternative　model　parameters.　The　development　of　a　calibration　framework　typically　involves　the　following:　(1)　definition　of　the　model　variables,　coefficients,　and　equations;　(2)　selection　of　an　objective　function　to　measure　the　quality　of　the　calibration;　(3)　selection　of　the　set　of　data　to　be　used　for　the　calibration　process;　and　(4)　selection　of　an　optimization/manual　scheme　for　altering　the　coefficient　values　in　the　direction　of　reducing　the　objective　function.　Hydraulic　calibration　usually　involves　the　modification　of　system　demands,　fine-tuning　the　roughness　values　of　pipes,　altering　pump　operation　characteristics,　and　adjusting　other　model　attributes　that　affect　simulation　results,　in　particular　those　that　have　significant　uncertainty　associated　with　their　values.　From　the　previous　steps,　it　is　clear　that　model　calibration　is　neither　unique　nor　a　straightforward　technical　task.　The　success　of　a　calibration　process　depends　on　the　modeler＇s　experience　and　intuition,　as　well　as　on　the　mathematical　model　and　procedures　adopted　for　the　calibration　process.　This　paper　provides　a　summary　of　the　Battle　of　the　Water　Calibration　Networks　(BWCN),　the　goal　of　which　was　to　objectively　compare　the　solutions　of　different　approaches　to　the　calibration　of　water　distribution　systems　through　application　to　a　real　water　distribution　system.　Fourteen　teams　from　academia,　water　utilities,　and　private　consultants　participated.　The　BWCN　outcomes　were　presented　and　assessed　at　the　12th　Water　Distribution　Systems　Analysis　conference　in　Tucson,　Arizona,　in　September　2010.　This　manuscript　summarizes　the　BWCN　exercise　and　suggests　future　research　directions　for　the　calibration　of　water　distribution　systems.　DOI:　10.1061/(ASCE)WR.1943-5452.0000191.　(C)　2012　American　Society　of　Civil　Engineers.