Photoacoustic　(PA)　imaging　as　a　fast-developing　imaging　technique　has　great　potential　in　biomedical　and　clinical　applications.　It　is　a　noninvasive　imaging　modality　that　depends　on　the　light-absorption　coefficient　of　the　imaged　tissue　and　the　injected　PA-imaging　contrast　agents.　Furthermore,　PA　imaging　provides　superb　contrast,　super　spatial　resolution,　and　high　penetrability　and　sensitivity　to　tissue　functional　characteristics　by　detecting　the　acoustic　wave　to　construct　PA　images.　In　recent　years,　a　series　of　PA-imaging　contrast　agents　are　developed　to　improve　the　PA-imaging　performance　in　biomedical　applications.　Here,　recent　progress　of　PA　contrast　agents　and　their　biomedical　applications　are　outlined.　PA　contrast　agents　are　classified　according　to　their　components　and　function,　and　gold　nanocrystals,　gold-nanocrystal　assembly,　transition-metal　chalcogenides/MXene-based　nanomaterials,　carbon-based　nanomaterials,　other　inorganic　imaging　agents,　small　organic　molecules,　semiconducting　polymer　nanoparticles,　and　nonlinear　PA-imaging　contrast　agents　are　discussed.　The　applications　of　PA　contrast　agents　as　biosensors　(in　the　sensing　of　metal　ions,　pH,　enzymes,　temperature,　hypoxia,　reactive　oxygen　species,　and　reactive　nitrogen　species)　and　in　bioimaging　(lymph　nodes,　vasculature,　tumors,　and　brain　tissue)　are　discussed　in　detail.　Finally,　an　outlook　on　the　future　research　and　investigation　of　PA-imaging　contrast　agents　and　their　significance　in　biomedical　research　is　presented.　©　2018　WILEY-VCH　Verlag　GmbH　&　Co.　KGaA,　Weinheim