An Event is a happening that affects the execution of a process. Events are used to model time, conditions and communication inbetween different processes.

There are three types of Events:

• Start Events,
• End Events and
• Intermediate Events.

Start Events

The occurance of the specified Start Event initiates the process (i.e. a new process instance).

The most commonly used Start Events are

• "Plain" Start Event, this does not actually define what triggers the start of the process.
• Start Message Event, an incoming message (E-Mail, letter or anything else that matches the abstraction of a message) initiates the process.
• Start Timer Event, to a given point of time or after a given period of time the process starts.
• Start Conditional Event, the process starts when a condition specified by a rule is fulfilled.

End Event

A final event is triggered as termination of the process.

Besides the "Plain" End Event there is especially the Message End Event which is used to express a message as notification about the termination of the process.

Note that the letter is black here, this indicates a Throwing Event. The distinction between Throwing and Catching Events was introduced with BPMN 1.2 and helps to clarify who sends Events and who receives Events. Start Events are caught (in this example a message is received) and End Events are thrown (a message is sent). The modeler can explicitly choose between Catching and Throwing Events only with Intermediate Events which are explained below.

Catching Intermediate Events

The process awaits an event (for example the incoming of a message) and stops until the event is "catched". An event can be caught when it is thrown somewhere else. Though an Intermediate Timer cannot be thrown, is can only be catched as the time is a somehow external influence to the process. Catching Events are white.

Throwing Intermediate Events

During the execution of a process Events can be thrown (for example messages can be sent). Throwing Intermediate Events are caught by Catching Intermediate Events as explained above but are processed asynchronously. This means the process of the Throwing Event does not stop until it is caught somewhere else.

Attached Intermediate Events

Intermediate Events can also be attached to Activities. If the attached Event occurs during the execution of the activity, the outgoing flow will be redirected.