Photoacoustic　(PA)　imaging　is　a　nonionizing,　noninvasive　imaging　technique　that　combines　optical　and　ultrasonic　imaging　modalities　to　provide　images　with　excellent　contrast,　spatial　resolution,　and　penetration　depth.　Exogenous　PA　contrast　agents　are　created　to　increase　the　sensitivity　and　specificity　of　PA　imaging　and　to　offer　diagnostic　information　for　illnesses.　The　existing　PA　contrast　agents　are　categorized　into　two　groups　in　this　review:　＂always-on＂　and　＂turn-on,＂　based　on　their　ability　to　be　triggered　by　target　molecules.　The　present　state　of　these　probes,　their　merits　and　limitations,　and　their　future　development,　is　explored.