Marketing cookies — also called advertising, targeting, or retargeting cookies — track visitors across sites to build ad profiles, retarget them, and measure campaign performance. Named examples: Meta's _fbp and fr, Google Ads' IDE and _gcl_au, LinkedIn's bcookie, plus TikTok, Twitter, and Pinterest tags. They always require opt-in consent under GDPR and ePrivacy Article 5(3) — never strictly necessary. Under CCPA and CPRA, sharing marketing cookie data with ad networks typically counts as "sale" or "sharing," triggering opt-out rights and GPC honoring. Marketing cookies are the number one target of CNIL, Garante, and ICO enforcement actions for pre-consent firing.