SysML (Systems Modeling Language)
SysML (Systems Modeling Language) is the graphical modelling language for systems engineering standardised by the OMG (Object Management Group). It describes requirements, structure and behaviour of complex systems and is the most widespread language for Model-Based Systems Engineering (MBSE).
Relationship to UML
SysML emerged from UML: SysML v1 is defined as a profile of UML 2. It reuses a subset of the UML diagrams, replaces software-specific concepts with discipline-neutral ones — the class is replaced by the block, which can equally represent hardware, software, mechanics, data or people — and adds what UML lacks for systems engineering: a dedicated requirement diagram and the parametric diagram for physical relationships and constraints.
This makes SysML suitable for what UML does not cover: describing complete technical systems in which software is only one of several disciplines.
The nine diagram types (SysML v1)
SysML v1 defines nine diagram types in three groups:
| Group | Diagram | Purpose |
|---|---|---|
| Structure | Block definition diagram (bdd) | System building blocks, their properties and hierarchy |
| Structure | Internal block diagram (ibd) | Internal structure of a block: parts, ports, connections |
| Structure | Parametric diagram | Physical equations and constraints between values |
| Structure | Package diagram | Organisation of the model into packages |
| Behaviour | Activity diagram | Processes and flows of material, energy and information |
| Behaviour | Sequence diagram | Interactions between system parts over time |
| Behaviour | State machine diagram | States and transitions of a block |
| Behaviour | Use case diagram | Usage scenarios and the actors involved |
| Requirements | Requirement diagram | Requirements and their relationships to model elements (satisfy, verify, derive) |
The requirement relationships in particular make SysML interesting for regulated development: they anchor traceability between requirements, architecture and verification directly in the model.
SysML v1 and SysML v2
With SysML v2, the OMG has adopted a fundamentally reworked successor. The most important differences: SysML v2 is no longer a UML profile but is based on its own, purpose-built metamodel (KerML). Alongside the graphical notation there is a textual notation on equal footing, which makes models versionable and diffable like source code. And a standardised API governs cross-tool access to models — a major step against vendor lock-in and towards vendor-independent integration of toolchains. For new programmes, SysML v2 is considered the strategically right choice; existing v1 models can be migrated.
SysML in practice
A language alone does not make MBSE. SysML defines what can be expressed — not how a team works with it. Without a tailored method (which model levels, which views, which level of detail), you quickly end up with models that are formally correct but practically worthless. In our experience, the biggest adoption hurdle is also not the tool but the SysML learning curve within the team: working with formal semantics takes practice. Those who accompany the introduction methodically and start with a pilot project, instead of converting the whole organisation at once, avoid the most common failures.