BPMN events explained
A BPMN event is something that happens, drawn as a circle. The border style shows where it sits in the flow — start (thin circle), intermediate (double circle), or end (thick circle) — and the icon inside shows what triggers it, such as a message, a timer, or an error. Events are how a BPMN diagram expresses "wait for," "react to," and "finish."
The three positions
| Event | Border | Role |
|---|---|---|
| Start | Thin single line | Begins the process (one trigger) |
| Intermediate | Double line | Happens during the process |
| End | Thick single line | Ends a path of the process |
A well-formed process has at least one start event and reaches at least one end event on every path.
Common trigger types
The icon inside the circle is the trigger:
- None (empty circle) — a plain start or end with no special trigger.
- Message (envelope) — sending or receiving something from another participant. A receiving message start event begins a process when a message arrives.
- Timer (clock) — waiting for a duration or a specific date/time ("every Monday", "after 24 hours").
- Error (lightning) — throwing or catching an error. Common as a boundary event on a task.
- Signal (triangle) — a broadcast that any number of processes can react to.
- Terminate (solid circle) — an end event that immediately stops the entire process, including parallel branches.
Throwing vs catching
Intermediate events come in two flavours:
- Catching (hollow icon) — the process waits for the event (e.g. wait to receive a message).
- Throwing (solid icon) — the process emits the event (e.g. send a message) and continues.
Boundary events — handling interruptions
A boundary event sits on the edge of a task and handles something that can interrupt it:
- Timer boundary event — "if this task isn't done in 2 days, escalate."
- Error boundary event — "if this task fails, route to the recovery path."
- Message boundary event — "if a cancellation arrives while this task runs, stop and cancel."
Boundary events are the cleanest way to model exceptions without cluttering the happy path — a hallmark of a mature diagram.
A practical example
An order process might use: a message start event (order received) → tasks → a timer boundary event on "Await payment" (cancel after 24h) → an end event (order shipped) and another end event (order cancelled). That single diagram now expresses both the normal flow and the timeout, unambiguously.
Modelling exceptions by hand is fiddly. If you describe the timeouts and failure paths in plain English, BPMN Studio will place the right event types for you. Continue with gateways explained, the full symbol reference, or how to create a BPMN diagram.
Frequently asked questions
What is an event in BPMN?
An event is something that happens during a process, drawn as a circle. Events start the process, occur along the way (intermediate), or end it. The circle's border shows the position and an inner icon shows the trigger, such as a message, timer or error.
What is the difference between start, intermediate and end events?
A start event (thin circle) begins the process; an intermediate event (double-line circle) happens during it — either waiting for something or throwing something; an end event (thick circle) marks where a path finishes.
What is a boundary event?
A boundary event is an intermediate event attached to the edge of a task. It handles something that can interrupt that task — for example, a timer boundary event that fires if the task takes too long, or an error boundary event that catches a failure and routes it down a different path.