Cross-device tracking in advertising is a crucial technique used to identify a single user across multiple digital devices. Its primary purpose is to create a unified profile of consumer behavior, enabling advertisers to deliver more consistent and personalized ad experiences regardless of the device being used. This process involves linking user activity on their smartphone, tablet, laptop, and desktop, allowing marketers to understand their complete journey and avoid showing repetitive or irrelevant advertisements. Common methods include deterministic matching, relying on shared logins (e.g., Google account), and probabilistic matching, which uses aggregated data points like IP addresses, Wi-Fi networks, and browser fingerprints. By understanding which devices belong to the same individual, advertisers can optimize ad delivery, improve campaign performance measurement, and build more effective attribution models, ultimately enhancing ROI.