diff --git a/planning/behavior_path_planner/docs/behavior_path_planner_interface_design.md b/planning/behavior_path_planner/docs/behavior_path_planner_interface_design.md
index 4072e09f5df3a..d401cbde5a16c 100644
--- a/planning/behavior_path_planner/docs/behavior_path_planner_interface_design.md
+++ b/planning/behavior_path_planner/docs/behavior_path_planner_interface_design.md
@@ -1,5 +1,178 @@
 # Interface design
 
+## State transition
+
+```plantuml
+@startuml
+title PlannerManager::run Method Execution Flow
+
+skinparam defaultTextAlignment center
+skinparam noteTextAlignment left
+skinparam backgroundColor #FFFFFF
+skinparam ArrowColor #333333
+skinparam ArrowFontColor #333333
+skinparam partitionBackgroundColor #FFCC66
+
+start
+if (root_lanelet_ is not set) then (yes)
+  :root_lanelet_ = updateRootLanelet(data);
+else (no)
+endif
+
+repeat
+  :Iterate over manager_ptrs_ to setData;
+repeat while (Any manager_ptr left)
+
+partition "Result Output" {
+  if (is_any_approved_module_running) then (yes)
+    if (is_any_candidate_module_running_or_idle) then (yes)
+      if (is_out_of_route) then (yes)
+        :output = createGoalAroundPath();
+        :generateCombinedDrivableArea(output, data);
+        stop
+      else (no)
+      endif
+    else (no)
+    endif
+  else (no)
+  endif
+
+  repeat
+    :approved_modules_output = runApprovedModules();
+    :request_modules = getRequestModules();
+    if (request_modules is empty) then (yes)
+      stop
+    else (no)
+    endif
+
+    :[highest_priority_module, candidate_modules_output] = runRequestModules();
+    if (highest_priority_module is not set) then (yes)
+      stop
+    else (no)
+    endif
+
+    if (highest_priority_module is waiting approval) then (yes)
+      stop
+    else (no)
+    endif
+
+    :addApprovedModule(highest_priority_module);
+    :clearCandidateModules();
+
+    if (iteration limit reached) then (yes)
+      stop
+    else (no)
+    endif
+  repeat while (Modules to process or iteration limit not reached)
+}
+
+repeat
+  :Iterate over manager_ptrs_ to updateObserver;
+repeat while (Any manager_ptr left)
+
+:generateCombinedDrivableArea(result_output, data);
+
+stop
+@enduml
+```
+
+```plantuml
+@startuml
+skinparam defaultTextAlignment center
+skinparam noteTextAlignment left
+skinparam backgroundColor #FFFFFF
+skinparam ArrowColor #333333
+skinparam ArrowFontColor #333333
+
+title Module State Update Flowchart
+
+' IDLE State logic
+partition #FFCC66 "IDLE State" {
+  start
+  if(current_state_ == ModuleStatus::IDLE ?) then (yes)
+    if(canTransitIdleToRunningState() ?) then (yes)
+      :return ModuleStatus::RUNNING;
+      stop
+    else (no)
+      :return ModuleStatus::IDLE;
+      stop
+    endif
+  else (no)
+  endif
+}
+
+' RUNNING State logic
+partition #66CC99 "RUNNING State" {
+  if(current_state_ == ModuleStatus::RUNNING ?) then (yes)
+    if(canTransitSuccessState() ?) then (yes)
+      :return ModuleStatus::SUCCESS;
+      stop
+    else (no)
+    endif
+
+    if(canTransitFailureState() ?) then (yes)
+      :return ModuleStatus::FAILURE;
+      stop
+    else (no)
+    endif
+
+    if(canTransitWaitingApprovalState() ?) then (yes)
+      :return ModuleStatus::WAITING_APPROVAL;
+      stop
+    else (no)
+      :return ModuleStatus::RUNNING;
+      stop
+    endif
+  else (no)
+  endif
+}
+
+' WAITING_APPROVAL State logic
+partition #6699CC "WAITING_APPROVAL State" {
+  if(current_state_ == ModuleStatus::WAITING_APPROVAL ?) then (yes)
+    if(canTransitSuccessState() ?) then (yes)
+      :return ModuleStatus::SUCCESS;
+      stop
+    else (no)
+    endif
+
+    if(canTransitFailureState() ?) then (yes)
+      :return ModuleStatus::FAILURE;
+      stop
+    else (no)
+    endif
+
+    if(canTransitWaitingApprovalToRunningState() ?) then (yes)
+      :return ModuleStatus::RUNNING;
+      stop
+    else (no)
+      :return ModuleStatus::WAITING_APPROVAL;
+      stop
+    endif
+  else (no)
+  endif
+}
+
+' Terminal States logic
+partition #99CC66 "Terminal States" {
+  if(current_state_ == ModuleStatus::SUCCESS ?) then (yes)
+    :return ModuleStatus::SUCCESS;
+    stop
+  else (no)
+  endif
+
+  if(current_state_ == ModuleStatus::FAILURE ?) then (yes)
+    :return ModuleStatus::FAILURE;
+    stop
+  else (no)
+  endif
+
+  :return ModuleStatus::IDLE;
+  stop
+}
+@enduml
+```
+
 !!! warning
 
     Under Construction