How do dynamic segments differ from static ones?

You built a "recent buyers" segment six months ago, exported a CSV of 2,000 contacts, and loaded it into your ESP. Today that segment still shows 2,000 people, most of whom bought ages ago and have long since moved on. That's a static segment doing exactly what static segments do: staying frozen while your audience keeps moving.

Static segments are fixed at the point of creation. They don't update automatically; you define a list once and it stays that way until you manually refresh it. They're useful for specific, time-bound situations: everyone who attended a webinar on a given date, contacts who were part of a migration batch, people who qualified for a one-time offer. The list is intentionally closed.

Dynamic segments work differently. They're defined by criteria rather than a fixed list, and they rebuild continuously based on who meets those criteria right now. "Anyone who clicked in the last 30 days" is a dynamic segment: tomorrow it includes whoever clicked today and drops anyone who hasn't clicked in the last 30 days. Klaviyo calls these live audiences, Mailchimp manages them through audience groups and tags, and HubSpot builds them as active lists. The terminology differs, but the behavior is the same: the segment reflects current state, not past state.

For ongoing programs like nurture sequences, win-back flows, and re-engagement campaigns, dynamic segments are almost always the right choice. You want to be sending to people who currently meet the criteria, not to whoever met it eight months ago. Static segments make sense for one-time campaigns, event-based sends, and any situation where the list definition is the point (an invitation list, a suppression list, a test group).

The practical question is whether your ESP updates dynamic segments in real time or on a schedule. Some platforms rebuild criteria-based audiences at send time; others sync on a daily or hourly cadence. If your "clicked in the last 30 days" segment refreshes once a day, it'll be slightly stale between updates. Check your segment refresh settings before relying on a dynamic segment for a time-sensitive campaign.