What is the ACCDB file format?
The .accdb format is the standard database file used by Microsoft Access 2007 and later. It replaced the older .mdb format and introduced the ACE (Access Connectivity Engine) as its underlying database engine - a successor to the Jet engine used in earlier versions.
Compared to .mdb, .accdb adds several important capabilities: data macros (event-driven logic attached directly to tables), multivalued fields, an Attachment field type for embedding files within records, and stronger 128-bit AES encryption. Integration with Microsoft Outlook and SharePoint was also expanded, though the Access Services publishing feature for SharePoint Online was subsequently retired by Microsoft.
Features present in .mdb that were deliberately dropped include user-level security and database replication - both considered legacy and superseded by more modern access-control approaches.
The file header carries a distinctive signature: bytes 00 01 00 00 followed by the ASCII string Standard ACE DB at offset 4, distinguishing it from .mdb files (which carry Standard Jet DB at the same offset). While the database is open, Access creates a companion .laccdb lock file alongside it.
Beyond Microsoft Access itself, .accdb files can be opened in read (and sometimes write) mode by LibreOffice Base and by MDB Tools on Linux. Compiled or locked variants include .accde (source code stripped), .accdt (template), and .accdr (runtime-mode copy).
Security & safety
RISK: MEDIUMAn .accdb can contain VBA macros and data macros that run code, so a database from an untrusted source can carry malware - Access opens unknown databases with macros disabled (the Trust Center / yellow security bar) and you should leave them disabled unless you trust the sender. Beyond macros, the data itself is inert. Never delete a .laccdb lock file while the database is open in another session. For old databases, note that .accdb files can be password-encrypted (Database Tools > Encrypt with Password); the encryption in ACCDB 2010+ is reasonably strong, earlier ACCDB 2007 encryption is weak.
Format details
in a nutshellPrograms that open ACCDB files
Windows4 apps
Microsoft Access
Paid Double-click the .accdb to open it. The native app (Microsoft 365 / Office); full read-write, forms, reports and VBA. 
MDB Viewer Plus
Free Portable freeware (no install) - open the .accdb to view/edit tables and export to Excel/CSV/PDF. Needs the Microsoft Access Database Engine (free) for ACCDB support. 
LibreOffice Base
Open-source Free office suite; connect to the .accdb to browse tables and run queries. Some forms/VBA won't carry over.
Microsoft Access Runtime
Free Free runtime that RUNS an Access app (forms/reports) but cannot design it. Use when you only need to use an existing database. 
macOS2 apps
LibreOffice Base
Open-source Best free Mac option - open the .accdb to view tables and export data. There is NO Microsoft Access for Mac. Complex queries/forms may not load.
Parallels Desktop + Windows
Paid Run Windows (and real Microsoft Access) in a VM on Mac when LibreOffice Base can't handle the database fully. 
Linux2 apps
LibreOffice Base
Open-source Open the .accdb to read tables and export. Combine with the mdbtools package (mdb-export) for command-line extraction.
MDB Tools
Open-source Command line: 'mdb-tables file.accdb' lists tables, 'mdb-export file.accdb TableName > out.csv' dumps to CSV. Technical details
deep spec| Database engine | Microsoft ACE (Access Connectivity Engine), introduced with Access 2007 as the Jet 4.0 successor |
| File signature | Hex `00 01 00 00` at offset 0; ASCII string `Standard ACE DB` starting at offset 4 |
| MIME type | `application/x-msaccess` |
| Maximum database size | 2 GB per `.accdb` file |
| Maximum fields per table | 255 fields |
| Multivalued fields | Supported - a single field can store multiple values; not available in `.mdb` |
| Attachment field type | Supported - embeds files directly inside records; not available in `.mdb` |
| Data macros | Supported - event-driven logic attached to table events such as After Insert or After Update; absent in `.mdb` |
| Encryption | 128-bit AES database-password encryption, replacing the weak XOR-based scheme in `.mdb` |
| Lock file | `.laccdb` companion file created automatically while the database is open to coordinate concurrent access |
| User-level security | Removed; was available in `.mdb` via a Workgroup Information File (`.mdw`) |
| Compiled variant | `.accde` - VBA source code stripped, distributable locked copy |
| Template variant | `.accdt` - Access Database Template for distributing pre-built database structures |
| Runtime variant | `.accdr` - opens in runtime mode, hiding the Navigation Pane and design views |
| Unicode support | Native UTF-16 storage for text fields |
| Third-party support | Read/write via LibreOffice Base; read-only access via MDB Tools (`mdb-export`, `mdb-tables`) on Linux |
| Released | 2007 (Access 2007, with the new ACE database engine) |
| Specification | learn.microsoft.com |
ACCDB conversions
Community Q&A
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