LSMW (Legacy System Migration Workbench) is a standard SAP tool used to migrate data from non-SAP (legacy) systems into SAP. It is mainly used during data migration projects such as SAP implementation, system upgrades, or when large volumes of master data (customers, vendors, materials) or transactional data (purchase orders, sales orders) need to be transferred.

It is typically used:

• During initial SAP go-live to load historical data.
• For one-time or periodic data loads where manual entry is not feasible.
• When a business needs mass uploads/updates without custom ABAP programs.

The LSMW execution process follows 14 guided steps (though not all are mandatory):

1. Maintain Object Attributes – Define project, subproject, and object.
2. Maintain Source Structures – Define data hierarchy.
3. Maintain Source Fields – List legacy fields.
4. Maintain Structure Relations – Map source to SAP structures.
5. Maintain Field Mapping and Conversion Rules – Map fields.
6. Maintain Fixed Values, Translations, User Routines – Apply conversion logic.
7. Specify Files – Upload legacy file (CSV/TXT).
8. Assign Files – Link files to structures.
9. Read Data – Load file into LSMW.
10. Display Read Data – Verify uploaded values.
11. Convert Data – Apply mapping rules.
12. Display Converted Data – Check converted output.
13. Create Batch Input Session – Generate posting session.
14. Run Batch Input Session – Execute session and post data into SAP.

 In short: Use LSMW when migrating or mass-loading data into SAP. Process = Define → Map → Upload → Read → Convert → Post.