diff --git a/milli/src/error.rs b/milli/src/error.rs
index 02740377c..c6dc85e9c 100644
--- a/milli/src/error.rs
+++ b/milli/src/error.rs
@@ -1,6 +1,9 @@
 use std::io;
 
 use crate::{DocumentId, FieldId};
+use heed::{MdbError, Error as HeedError};
+
+pub type Object = serde_json::Map<String, serde_json::Value>;
 
 pub enum Error {
     InternalError(InternalError),
@@ -12,6 +15,16 @@ pub enum InternalError {
     DatabaseMissingEntry(DatabaseMissingEntry),
     FieldIdMapMissingEntry(FieldIdMapMissingEntry),
     IndexingMergingKeys(IndexingMergingKeys),
+    SerializationError(SerializationError),
+    StoreError(MdbError),
+    InvalidDatabaseTyping,
+    DatabaseClosing,
+}
+
+pub enum SerializationError {
+    Decoding { db_name: Option<&'static str> },
+    Encoding { db_name: Option<&'static str> },
+    InvalidNumberSerialization,
 }
 
 pub enum IndexingMergingKeys {
@@ -38,5 +51,40 @@ pub enum DatabaseMissingEntry {
 }
 
 pub enum UserError {
-
+    AttributeLimitReached,
+    DocumentLimitReached,
+    InvalidCriterionName { name: String },
+    InvalidDocumentId { document_id: DocumentId },
+    MissingDocumentId { document: Object },
+    MissingPrimaryKey,
+    DatabaseSizeReached,
+    NoSpaceLeftOnDevice,
+    InvalidStoreFile,
+}
+
+impl From<io::Error> for Error {
+    fn from(error: io::Error) -> Error {
+        // TODO must be improved and more precise
+        Error::IoError(error)
+    }
+}
+
+impl From<HeedError> for Error {
+    fn from(error: HeedError) -> Error {
+        use self::Error::*;
+        use self::InternalError::*;
+        use self::SerializationError::*;
+        use self::UserError::*;
+
+        match error {
+            HeedError::Io(error) => Error::from(error),
+            HeedError::Mdb(MdbError::MapFull) => UserError(DatabaseSizeReached),
+            HeedError::Mdb(MdbError::Invalid) => UserError(InvalidStoreFile),
+            HeedError::Mdb(error) => InternalError(StoreError(error)),
+            HeedError::Encoding => InternalError(SerializationError(Encoding { db_name: None })),
+            HeedError::Decoding => InternalError(SerializationError(Decoding { db_name: None })),
+            HeedError::InvalidDatabaseTyping => InternalError(InvalidDatabaseTyping),
+            HeedError::DatabaseClosing => InternalError(DatabaseClosing),
+        }
+    }
 }