diff --git a/ladybug_comfort/et.py b/ladybug_comfort/et.py
new file mode 100644
index 00000000..023845df
--- /dev/null
+++ b/ladybug_comfort/et.py
@@ -0,0 +1,54 @@
+# -*- coding: utf-8 -*-
+"""Utility functions to calculate Effective Temperature"""
+
+
+def effective_temperature_conditions(tre, te):
+    """ Inputs:
+        * tre: Radiant effective temperature
+        * te: Thermal emission
+        Outputs:
+        * tre: Radiant effective temperature
+        * te: Effective temperature
+    """
+    if tre < 1:
+        effectTE = -4
+    elif tre >= 1 and te < 9:
+        effectTE = -3
+    elif tre >= 9 and te < 17:
+        effectTE = -2
+    elif tre >= 17 and te < 21:
+        effectTE = -1
+    elif tre >= 21 and te < 23:
+        effectTE = 0
+    elif tre >= 23 and te < 27:
+        effectTE = 1
+    elif tre >= 27:
+        effectTE = 2
+
+    return tre, effectTE
+
+
+def effective_temperature(ta, ws, rh, sr, ac):
+    """ Inputs: 
+            * ta: Air Temperature 
+            * ws: Wind Speed
+            * rh: Relative Humidity
+            * sr: Solar Radiation
+            * ac: Air Flow
+        Outputs:
+            * tre: Radiant Effective Temperature 
+            * effectTE
+            * Comfort: boolean value for whether or not the effective temp is comfortable
+    """
+    if ws <= 0.2:
+        # formula by Missenard
+        te = ta - 0.4 * (ta - 10) * (1 - rh / 100)
+    elif ws > 0.2:
+        # modified formula by Gregorczuk (WMO, 1972; Hentschel, 1987)
+        te = 37 - ((37 - ta) / (0.68 - (0.0014 * rh) + (1 / (1.76 + 1.4 * (ws ** 0.75))))) - \
+            (0.29 * ta * (1 - 0.01 * rh))
+
+    # Radiative-effective temperature
+    tre = te + ((1 - 0.01 * ac) * sr) * ((0.0155 - 0.00025 * te) - (0.0043 - 0.00011 * te))
+
+    return effective_temperature_conditions(tre, te)
diff --git a/requirements.txt b/requirements.txt
index 61ff1678..79e84036 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1 +1,2 @@
 ladybug-core==0.43.18
+numpy
diff --git a/tests/et_test.py b/tests/et_test.py
new file mode 100644
index 00000000..2d0ee163
--- /dev/null
+++ b/tests/et_test.py
@@ -0,0 +1,16 @@
+# coding utf-8
+import pytest
+from ladybug_comfort.et import effective_temperature
+
+def test_effective_temperature():
+    """Test the effective_temperature function"""
+    et_obj = effective_temperature(
+        ta=13.7,
+        ws=7.8,
+        rh=74,
+        sr=104,
+        ac=318)
+
+    assert et_obj[0] == 0.07758930406978726
+    assert et_obj[1] == -4
+   
\ No newline at end of file