diff --git a/.DS_Store b/.DS_Store
index 36728efc..55899226 100644
Binary files a/.DS_Store and b/.DS_Store differ
diff --git a/pypfopt/expected_returns.py b/pypfopt/expected_returns.py
index 42a90ca4..59d24080 100644
--- a/pypfopt/expected_returns.py
+++ b/pypfopt/expected_returns.py
@@ -23,6 +23,7 @@
 import warnings
 import pandas as pd
 import numpy as np
+import math
 
 
 def returns_from_prices(prices, log_returns=False):
@@ -256,3 +257,36 @@ def capm_return(
 
     # CAPM formula
     return risk_free_rate + betas * (mkt_mean_ret - risk_free_rate)
+
+
+def calculate_downside_diviation( historical_returns, mar):
+    """
+    Calculate the downside diviation of all assets in a portfolio
+    
+    :param historical_returns: historical returns of assets in dataframe.
+    :type historical_returns: np.ndarray
+    :param mar: Minimum Acceptable Return. Preffered practices include either US 13-week T-bill or zero. 
+     CAUTION: mar must be in the same period as the returns. If you give daily returns then you need to convert mar to daily value.
+    :type mar: float
+    :param return_Max: an option to return the Max sortino ratio of the portfolio instead of a list of all sortino ratios for all stocks. Defaults to False 
+    :type return_Max: Boolean
+    :return: Sortino ratio
+    :rtype: float
+    """
+    downsideDiviations = []
+    linesOfData = len(historical_returns.iloc[:, [0]])
+    for stock in range(len(historical_returns.columns)):
+        noDataLines = 0 # counts NaN lines in dataset if any
+        negativeReturns = [] # stores the negative daily returns
+        for row in range(linesOfData):
+            if ( math.isnan(historical_returns.iloc[row, [stock]][0]) ):
+                noDataLines += 1
+                continue
+            if (  (historical_returns.iloc[row, [stock]][0] - mar) < 0 ):
+                negativeReturns.append(historical_returns.iloc[row, [stock][0]] - mar)
+        period = linesOfData - noDataLines # number of actual observations
+        squaredReturns = [r ** 2 for r in negativeReturns]
+        ts = sum(squaredReturns)
+        dd = math.sqrt(ts / period) * math.sqrt(period)
+        downsideDiviations.append(dd)
+    return downsideDiviations
\ No newline at end of file
diff --git a/pypfopt/objective_functions.py b/pypfopt/objective_functions.py
index 32c73661..835e05f1 100644
--- a/pypfopt/objective_functions.py
+++ b/pypfopt/objective_functions.py
@@ -115,6 +115,8 @@ def sharpe_ratio(w, expected_returns, cov_matrix, risk_free_rate=0.02, negative=
     return _objective_value(w, sign * sharpe)
 
 
+
+
 def L2_reg(w, gamma=1):
     r"""
     L2 regularisation, i.e :math:`\gamma ||w||^2`, to increase the number of nonzero weights.
@@ -224,3 +226,32 @@ def ex_post_tracking_error(w, historic_returns, benchmark_returns):
     mean = cp.sum(x_i) / len(benchmark_returns)
     tracking_error = cp.sum_squares(x_i - mean)
     return _objective_value(w, tracking_error)
+
+
+def sortino_ratio(w, expected_returns, downside_diviations, risk_free_rate = 0.02):
+    """
+    Calculate the Sortino ratio of a portfolio
+
+    :param w: asset weights in the portfolio
+    :type w: np.ndarray OR cp.Variable
+    :param expected_returns: expected return of each asset
+    :type expected_returns: np.ndarray
+    :param downside_diviations: The downside diviation of each asset
+    :type downside_diviations: np.ndarray
+    :param risk_free_rate: risk-free rate of borrowing/lending, defaults to 0.02.
+                           The period of the risk-free rate should correspond to the
+                           frequency of expected returns.
+    :type risk_free_rate: float, optional
+    :return: Sortino ratio
+    :rtype: float
+    """
+    mu =  w @ expected_returns
+    if isinstance(downside_diviations, list):
+        x = np.array(downside_diviations)
+        dd = w * x
+        sortino = (mu - risk_free_rate) / dd
+        return sum((w * sortino))
+    else:
+        dd = w @ downside_diviations
+        sortino = (mu - risk_free_rate) / dd
+        return _objective_value(w, sortino)