diff --git a/configs/mainnet.yaml b/configs/mainnet.yaml
index aa1858b69..6f2f582fa 100644
--- a/configs/mainnet.yaml
+++ b/configs/mainnet.yaml
@@ -3,6 +3,12 @@
 # Extends the mainnet preset
 PRESET_BASE: 'mainnet'
 
+# Transition
+# ---------------------------------------------------------------
+# TBD, 2**256-1 is a placeholder
+TERMINAL_TOTAL_DIFFICULTY: 115792089237316195423570985008687907853269984665640564039457584007913129639935
+
+
 # Genesis
 # ---------------------------------------------------------------
 # `2**14` (= 16,384)
diff --git a/configs/minimal.yaml b/configs/minimal.yaml
index 9f5cdbe32..8da3260f5 100644
--- a/configs/minimal.yaml
+++ b/configs/minimal.yaml
@@ -3,6 +3,12 @@
 # Extends the minimal preset
 PRESET_BASE: 'minimal'
 
+# Transition
+# ---------------------------------------------------------------
+# TBD, 2**256-1 is a placeholder
+TERMINAL_TOTAL_DIFFICULTY: 115792089237316195423570985008687907853269984665640564039457584007913129639935
+
+
 # Genesis
 # ---------------------------------------------------------------
 # [customized]
diff --git a/specs/merge/beacon-chain.md b/specs/merge/beacon-chain.md
index a9eea2f4a..03bf01155 100644
--- a/specs/merge/beacon-chain.md
+++ b/specs/merge/beacon-chain.md
@@ -81,7 +81,7 @@ This patch adds transaction execution to the beacon chain as part of the Merge f
 
 | Name | Value |
 | - | - |
-| `TERMINAL_TOTAL_DIFFICULTY` | `uint256(2**256 - 1)` |
+| `TERMINAL_TOTAL_DIFFICULTY` | **TBD** |
 
 ## Containers