# Execution
This page explains what happens under the hood when ZenML executes steps in static and dynamic pipelines. Regardless of where or how a step executes (inline or in an isolated environment, synchronous or concurrent), ZenML applies the same core semantics: inputs are loaded via materializers, outputs are materialized as versioned artifacts, lineage/metadata and logs are recorded, caching policies are respected, and step/run status is published consistently.
## Static pipelines
In static pipelines, ZenML executes the pipeline function before running the pipeline to compile a DAG of steps, which the orchestrator then schedules according to their upstream dependencies. This pre-compilation allows ZenML to optimize execution order and validate the DAG structure before any steps run.
### Execution scenarios
 
## Dynamic pipelines
[Dynamic pipelines](/concepts/steps_and_pipelines/dynamic_pipelines.md) execute the pipeline function at runtime. Each step executed inside the pipeline function can be:
* **Inline** (runs inside the orchestration environment)
* **Isolated** (runs in a separate environment via the orchestrator or a step operator)
And each step call can be:
* **Synchronous** (via `my_step(...)`): blocks until completion and returns the step output artifacts.
* **Concurrent** (via `my_step.submit(...)`): starts step execution in a separate thread and returns a future. The pipeline function resumes execution immediately.
### Execution scenarios
#### Synchronous inline
The step runs in-process inside the orchestration environment. The pipeline function blocks until the step completes.
#### Concurrent inline
The step runs in-process in a separate thread. The pipeline function continues immediately and only waits when results are consumed.
#### Synchronous isolated
The step runs in a separate environment (via the orchestrator or step operator). The pipeline function blocks until the job completes.
#### Concurrent isolated
The step runs in a separate environment (via the orchestrator or step operator). The pipeline function continues immediately and only waits when results are consumed.
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