# Message Handling Pipeline

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NServiceBus uses the concept of _pipelines_. A pipeline refers to the series of actions taken as a result of the triggering action.

For example:

* The incoming pipeline is triggered due to an incoming message
* The outgoing pipeline is triggered due to an outgoing message
* The [recoverability](/nservicebus/recoverability/index.md) pipeline is triggered due to failure during processing


## Pipeline customization

There are several ways to customize the pipelines with varying levels of complexity.

* [Manipulate the pipeline with behaviors](/nservicebus/pipeline/manipulate-with-behaviors.md)
* [Steps, stages and connectors](/nservicebus/pipeline/steps-stages-connectors.md)
* [Message mutators](/nservicebus/pipeline/message-mutators.md)
* [Abort the pipeline](/nservicebus/pipeline/aborting.md)
* [React to pipeline events](/nservicebus/pipeline/events.md)

Unit testing of custom extensions is supported by [the `NServiceBus.Testing` library](/nservicebus/testing/index.md#testing-a-behavior).