"Unassigned" shows up in GA4's acquisition and attribution reports when the platform lacks sufficient data to credit a conversion to a known source. You'll see it most often in the "Session source/medium" dimension, sometimes accounting for 5-15% of conversions depending on your tracking setup. The main causes are session timeouts (default 30 minutes of inactivity), where a user returns after the session expires and converts without a new referrer. Direct traffic that converts after the attribution window also gets marked unassigned. Stripped UTM parameters—common when users copy-paste URLs or when privacy tools remove tracking codes—create the same issue. Cross-device journeys where GA4 can't stitch the user path together will show unassigned attribution. GA4's data-driven attribution model tries to fill these gaps using machine learning, but when there's genuinely no trackable source in the lookback window (default 30 days for conversions, 90 days for some properties), it defaults to unassigned rather than guessing. At Ottawa SEO, we see unassigned spike when clients have aggressive session timeout settings, lack proper UTM tagging on paid campaigns, or run offline-to-online funnels where the initial touchpoint isn't tracked. E-commerce sites with long consideration cycles (two weeks-plus between first visit and purchase) tend to show higher unassigned percentages than lead-gen sites with same-session conversions. To reduce unassigned traffic, extend your session timeout to 60-90 minutes for high-engagement sites, implement consistent UTM parameters across all paid and email campaigns, and use GA4's User-ID feature to track logged-in users across devices. Check your referral exclusion list—overly aggressive exclusions can strip attribution data. For businesses with significant offline touchpoints (phone calls, in-store visits), consider implementing call tracking with dynamic number insertion or adding UTM-tagged QR codes to bridge the gap. Expect 3-8% unassigned in well-tracked properties; anything above 12% signals a tagging or attribution configuration problem worth investigating.