diff --git a/beacon_chain/validation/src/block_validation.rs b/beacon_chain/validation/src/block_validation.rs
index 3c3b828ed..08aae7c0e 100644
--- a/beacon_chain/validation/src/block_validation.rs
+++ b/beacon_chain/validation/src/block_validation.rs
@@ -53,6 +53,7 @@ pub enum SszBlockValidationError {
     UnknownPoWChainRef,
     UnknownParentHash,
     BadAttestationSsz,
+    ParentSlotHigherThanBlockSlot,
     AttestationValidationError(AttestationValidationError),
     AttestationSignatureFailed,
     ProposerAttestationHasObliqueHashes,
@@ -204,6 +205,15 @@ impl<T> BlockValidationContext<T>
             }
         };
 
+        /*
+         * The parent block slot must be less than the block slot.
+         *
+         * In other words, the parent must come before the child.
+         */
+        if parent_block_slot >= block_slot {
+            return Err(SszBlockValidationError::ParentSlotHigherThanBlockSlot);
+        }
+
         /*
          * Generate the context in which attestations will be validated.
          */
diff --git a/beacon_chain/validation/tests/block_validation/tests.rs b/beacon_chain/validation/tests/block_validation/tests.rs
index 9e16756ec..3046998a9 100644
--- a/beacon_chain/validation/tests/block_validation/tests.rs
+++ b/beacon_chain/validation/tests/block_validation/tests.rs
@@ -94,6 +94,22 @@ fn test_block_validation_valid_known_block() {
     assert_eq!(status.unwrap(), (BlockStatus::KnownBlock, None));
 }
 
+#[test]
+fn test_block_validation_parent_slot_too_high() {
+    let params = get_simple_params();
+
+    let mutator = |mut block: Block, attester_map, proposer_map, stores| {
+        block.slot_number = params.validation_context_justified_slot + 1;
+        (block, attester_map, proposer_map, stores)
+    };
+
+    let status = run_block_validation_scenario(
+        &params,
+        mutator);
+
+    assert_eq!(status, Err(SszBlockValidationError::ParentSlotHigherThanBlockSlot));
+}
+
 #[test]
 fn test_block_validation_invalid_future_slot() {
     let params = get_simple_params();