diff --git a/mysticeti.nim b/mysticeti.nim
index f34667a..224ece3 100644
--- a/mysticeti.nim
+++ b/mysticeti.nim
@@ -42,6 +42,10 @@ export blocks.round
 export blocks.parents
 export blocks.id
 
+import ./mysticeti/basics/immutableseq
+
+export immutableseq
+
 import ./mysticeti/blocks/signed
 
 export signed.SignedBlock
diff --git a/mysticeti/basics.nim b/mysticeti/basics.nim
index f91434b..8dab6cd 100644
--- a/mysticeti/basics.nim
+++ b/mysticeti/basics.nim
@@ -11,3 +11,7 @@ export results
 import ./dependencies
 
 export dependencies
+
+import ./basics/immutableseq
+
+export immutableseq
diff --git a/mysticeti/basics/immutableseq.nim b/mysticeti/basics/immutableseq.nim
new file mode 100644
index 0000000..481eb07
--- /dev/null
+++ b/mysticeti/basics/immutableseq.nim
@@ -0,0 +1,27 @@
+type ImmutableSeq*[Element] = ref object
+  ## Encapsulates a sequence, so that it no longer can be
+  ## modified, and can be passed by reference to avoid copying.
+  elements: seq[Element]
+
+func immutable*[Element](sequence: seq[Element]): ImmutableSeq[Element] =
+  ImmutableSeq[Element](elements: sequence)
+
+func copy*(sequence: ImmutableSeq): auto =
+  sequence.elements
+
+iterator items*(sequence: ImmutableSeq): auto =
+  for element in sequence.elements:
+    yield element
+
+func len*(sequence: ImmutableSeq): int =
+  sequence.elements.len
+
+func `[]`*(sequence: ImmutableSeq, index: int): auto =
+  sequence.elements[index]
+
+func contains*[Element](sequence: ImmutableSeq[Element], element: Element): bool =
+  sequence.elements.contains(element)
+
+func `==`*(a, b: ImmutableSeq): bool =
+  a.elements == b.elements
+
diff --git a/mysticeti/blocks/blck.nim b/mysticeti/blocks/blck.nim
index 82cf585..4074c52 100644
--- a/mysticeti/blocks/blck.nim
+++ b/mysticeti/blocks/blck.nim
@@ -3,12 +3,12 @@ import ../committee
 import ./blockid
 
 type
-  Block*[Dependencies] = object
+  Block*[Dependencies] = ref object
     id: BlockId[Dependencies]
     author: CommitteeMember
     round: uint64
-    parents: seq[BlockId[Dependencies]]
-    transactions: seq[Dependencies.Transaction]
+    parents: ImmutableSeq[BlockId[Dependencies]]
+    transactions: ImmutableSeq[Dependencies.Transaction]
 
 func calculateId(blck: var Block) =
   mixin hash
@@ -27,8 +27,8 @@ func new*[Dependencies](
   var blck = Block[Dependencies](
     author: author,
     round: round,
-    parents: parents,
-    transactions: transactions
+    parents: parents.immutable,
+    transactions: transactions.immutable
   )
   blck.calculateId()
   blck
diff --git a/mysticeti/validator.nim b/mysticeti/validator.nim
index 1a37c53..afa7001 100644
--- a/mysticeti/validator.nim
+++ b/mysticeti/validator.nim
@@ -153,7 +153,7 @@ func updateIndirect(validator: Validator, slot: ProposerSlot, round: Round) =
     return
   without anchorProposal =? anchor.proposal:
     return
-  var todo = anchorProposal.blck.parents
+  var todo = anchorProposal.blck.parents.copy
   while todo.len > 0:
     let parent = todo.pop()
     if parent.round < round.number + 2:
@@ -164,7 +164,7 @@ func updateIndirect(validator: Validator, slot: ProposerSlot, round: Round) =
         return
       without parentBlock =? round.find(parent):
         raiseAssert "parent block not found"
-      todo.add(parentBlock.blck.parents)
+      todo.add(parentBlock.blck.parents.copy)
   slot.skip()
 
 iterator committed*(validator: Validator): auto =
diff --git a/tests/mysticeti/testBlocks.nim b/tests/mysticeti/testBlocks.nim
index e76a738..5016007 100644
--- a/tests/mysticeti/testBlocks.nim
+++ b/tests/mysticeti/testBlocks.nim
@@ -19,8 +19,8 @@ suite "Blocks":
     let blck = Block.new(author, round, parents, transactions)
     check blck.author == author
     check blck.round == round
-    check blck.parents == parents
-    check blck.transactions == blck.transactions
+    check blck.parents == parents.immutable
+    check blck.transactions == transactions.immutable
 
   test "blocks have an id consisting of author, round and hash":
     let blck = Block.example