diff --git a/routes/simulations.py b/routes/simulations.py
index 0a9f100..162533f 100644
--- a/routes/simulations.py
+++ b/routes/simulations.py
@@ -1,9 +1,15 @@
-from fastapi import APIRouter, HTTPException, Depends
-from typing import List
+from typing import List, Optional, cast
 
-from services.simulation import run_simulation
+from fastapi import APIRouter, Depends, HTTPException, status
+from pydantic import BaseModel, PositiveInt
 from sqlalchemy.orm import Session
+
 from config.database import SessionLocal
+from models.parameters import Parameter
+from models.scenario import Scenario
+from models.simulation_result import SimulationResult
+from services.reporting import generate_report
+from services.simulation import run_simulation
 
 router = APIRouter(prefix="/api/simulations", tags=["Simulations"])
 
@@ -16,10 +22,109 @@ def get_db():
         db.close()
 
 
-@router.post("/run", response_model=List[dict])
-async def simulate(params: List[dict], iterations: int = 1000, db: Session = Depends(get_db)):
-    if not params:
-        raise HTTPException(status_code=400, detail="No parameters provided")
-    # TODO: you might use db to fetch scenario info or persist results
-    results = run_simulation(params, iterations)
-    return results
+class SimulationParameterInput(BaseModel):
+    name: str
+    value: float
+    distribution: Optional[str] = "normal"
+    std_dev: Optional[float] = None
+    min: Optional[float] = None
+    max: Optional[float] = None
+    mode: Optional[float] = None
+
+
+class SimulationRunRequest(BaseModel):
+    scenario_id: int
+    iterations: PositiveInt = 1000
+    parameters: Optional[List[SimulationParameterInput]] = None
+    seed: Optional[int] = None
+
+
+class SimulationResultItem(BaseModel):
+    iteration: int
+    result: float
+
+
+class SimulationRunResponse(BaseModel):
+    scenario_id: int
+    iterations: int
+    results: List[SimulationResultItem]
+    summary: dict
+
+
+def _load_parameters(db: Session, scenario_id: int) -> List[SimulationParameterInput]:
+    db_params = (
+        db.query(Parameter)
+        .filter(Parameter.scenario_id == scenario_id)
+        .order_by(Parameter.id)
+        .all()
+    )
+    return [
+        SimulationParameterInput(
+            name=cast(str, item.name),
+            value=cast(float, item.value),
+        )
+        for item in db_params
+    ]
+
+
+@router.post("/run", response_model=SimulationRunResponse)
+async def simulate(payload: SimulationRunRequest, db: Session = Depends(get_db)):
+    scenario = db.query(Scenario).filter(
+        Scenario.id == payload.scenario_id).first()
+    if scenario is None:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail="Scenario not found",
+        )
+
+    parameters = payload.parameters or _load_parameters(
+        db, payload.scenario_id)
+    if not parameters:
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail="No parameters provided",
+        )
+
+    raw_results = run_simulation(
+        [param.dict(exclude_none=True) for param in parameters],
+        iterations=payload.iterations,
+        seed=payload.seed,
+    )
+
+    if not raw_results:
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail="Simulation produced no results",
+        )
+
+    # Persist results (replace existing values for scenario)
+    db.query(SimulationResult).filter(
+        SimulationResult.scenario_id == payload.scenario_id
+    ).delete()
+    db.bulk_save_objects(
+        [
+            SimulationResult(
+                scenario_id=payload.scenario_id,
+                iteration=item["iteration"],
+                result=item["result"],
+            )
+            for item in raw_results
+        ]
+    )
+    db.commit()
+
+    summary = generate_report(raw_results)
+
+    response = SimulationRunResponse(
+        scenario_id=payload.scenario_id,
+        iterations=payload.iterations,
+        results=[
+            SimulationResultItem(
+                iteration=int(item["iteration"]),
+                result=float(item["result"]),
+            )
+            for item in raw_results
+        ],
+        summary=summary,
+    )
+    return response
diff --git a/services/simulation.py b/services/simulation.py
index db4a4ed..4a433f2 100644
--- a/services/simulation.py
+++ b/services/simulation.py
@@ -1,17 +1,140 @@
-from typing import Dict, List
+from __future__ import annotations
+
+from dataclasses import dataclass
+from random import Random
+from typing import Dict, List, Literal, Optional, Sequence
 
 
-def run_simulation(parameters: List[Dict[str, float]], iterations: int = 1000) -> List[Dict[str, float]]:
-    """
-    Run Monte Carlo simulation with given parameters.
+DEFAULT_STD_DEV_RATIO = 0.1
+DEFAULT_UNIFORM_SPAN_RATIO = 0.15
+DistributionType = Literal["normal", "uniform", "triangular"]
 
-    Args:
-        parameters: List of parameter dicts with keys 'name' and 'value'.
-        iterations: Number of simulation iterations.
 
-    Returns:
-        List of simulation result dicts for each iteration.
-    """
-    # TODO: implement Monte Carlo logic
+@dataclass
+class SimulationParameter:
+    name: str
+    base_value: float
+    distribution: DistributionType
+    std_dev: Optional[float] = None
+    minimum: Optional[float] = None
+    maximum: Optional[float] = None
+    mode: Optional[float] = None
+
+
+def _ensure_positive_span(span: float, fallback: float) -> float:
+    return span if span and span > 0 else fallback
+
+
+def _compile_parameters(parameters: Sequence[Dict[str, float]]) -> List[SimulationParameter]:
+    compiled: List[SimulationParameter] = []
+    for index, item in enumerate(parameters):
+        if "value" not in item:
+            raise ValueError(
+                f"Parameter at index {index} must include 'value'")
+        name = str(item.get("name", f"param_{index}"))
+        base_value = float(item["value"])
+        distribution = str(item.get("distribution", "normal")).lower()
+        if distribution not in {"normal", "uniform", "triangular"}:
+            raise ValueError(
+                f"Parameter '{name}' has unsupported distribution '{distribution}'"
+            )
+
+        span_default = abs(base_value) * DEFAULT_UNIFORM_SPAN_RATIO or 1.0
+
+        if distribution == "normal":
+            std_dev = item.get("std_dev")
+            std_dev_value = float(std_dev) if std_dev is not None else abs(
+                base_value) * DEFAULT_STD_DEV_RATIO or 1.0
+            compiled.append(
+                SimulationParameter(
+                    name=name,
+                    base_value=base_value,
+                    distribution="normal",
+                    std_dev=_ensure_positive_span(std_dev_value, 1.0),
+                )
+            )
+            continue
+
+        minimum = item.get("min")
+        maximum = item.get("max")
+        if minimum is None or maximum is None:
+            minimum = base_value - span_default
+            maximum = base_value + span_default
+        minimum = float(minimum)
+        maximum = float(maximum)
+        if minimum >= maximum:
+            raise ValueError(
+                f"Parameter '{name}' requires 'min' < 'max' for {distribution} distribution"
+            )
+
+        if distribution == "uniform":
+            compiled.append(
+                SimulationParameter(
+                    name=name,
+                    base_value=base_value,
+                    distribution="uniform",
+                    minimum=minimum,
+                    maximum=maximum,
+                )
+            )
+        else:  # triangular
+            mode = item.get("mode")
+            if mode is None:
+                mode = base_value
+            mode_value = float(mode)
+            if not (minimum <= mode_value <= maximum):
+                raise ValueError(
+                    f"Parameter '{name}' mode must be within min/max bounds for triangular distribution"
+                )
+            compiled.append(
+                SimulationParameter(
+                    name=name,
+                    base_value=base_value,
+                    distribution="triangular",
+                    minimum=minimum,
+                    maximum=maximum,
+                    mode=mode_value,
+                )
+            )
+    return compiled
+
+
+def _sample_parameter(rng: Random, param: SimulationParameter) -> float:
+    if param.distribution == "normal":
+        assert param.std_dev is not None
+        return rng.normalvariate(param.base_value, param.std_dev)
+    if param.distribution == "uniform":
+        assert param.minimum is not None and param.maximum is not None
+        return rng.uniform(param.minimum, param.maximum)
+    # triangular
+    assert (
+        param.minimum is not None
+        and param.maximum is not None
+        and param.mode is not None
+    )
+    return rng.triangular(param.minimum, param.maximum, param.mode)
+
+
+def run_simulation(
+    parameters: Sequence[Dict[str, float]],
+    iterations: int = 1000,
+    seed: Optional[int] = None,
+) -> List[Dict[str, float]]:
+    """Run a lightweight Monte Carlo simulation using configurable distributions."""
+
+    if iterations <= 0:
+        return []
+
+    compiled_params = _compile_parameters(parameters)
+    if not compiled_params:
+        return []
+
+    rng = Random(seed)
     results: List[Dict[str, float]] = []
+    for iteration in range(1, iterations + 1):
+        total = 0.0
+        for param in compiled_params:
+            sample = _sample_parameter(rng, param)
+            total += sample
+        results.append({"iteration": iteration, "result": total})
     return results