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diff --git a/lecture03/notes_03.ipynb b/lecture03/notes_03.ipynb
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@@ -268,6 +268,21 @@
     "# Print just the first few outputs\n",
     "print(dense1.output[:5])\n"
    ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "# Activation Function: ReLU\n",
+    "Rectified Linear Unit\n"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": []
   }
  ],
  "metadata": {
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diff --git a/lecture03/notes_03.py b/lecture03/notes_03.py
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+++ b/lecture03/notes_03.py
@@ -0,0 +1,174 @@
+# %% [markdown]
+# # Layer of Neurons and Batch of Data Using Numpy
+# From lecture 1, 1 layer of 3 neurons
+
+# %%
+import numpy as np
+
+inputs = [ # Batch of inputs
+    [1.0, 2.0, 3.0, 2.5], 
+    [2.0, 5.0, -1.0, 2.0], 
+    [-1.5, 2.7, 3.3, -0.8]
+]
+weights = np.array([
+    [0.2, 0.8, -0.5, 1],
+    [0.5, -0.91, 0.26, -0.5],
+    [-0.26, -0.27, 0.17, 0.87]
+])
+biases = [2.0, 3.0, 0.5]
+
+outputs = np.dot(inputs, weights.T) + biases
+# For every row of inputs, compute the dot of input set and weights
+print(outputs)
+
+# %% [markdown]
+# # 2 Layers and Batch of Data Using Numpy
+# 2 layers. Layer 1 and layer 2 have 3 neurons. Therefore, there are 3 final outputs.
+# 
+# There are 3 batches of input data to generate 3 total output arrays.
+
+# %%
+import numpy as np
+
+inputs = [[1, 2, 3, 2.5],
+          [2., 5., -1., 2],
+          [-1.5, 2.7, 3.3, -0.8]]
+
+weights = [[0.2, 0.8, -0.5, 1],
+           [0.5, -0.91, 0.26, -0.5],
+           [-0.26, -0.27, 0.17, 0.87]]
+
+biases = [2, 3, 0.5]
+
+weights2 = [[0.1, -0.14, 0.5],
+            [-0.5, 0.12, -0.33],
+            [-0.44, 0.73, -0.13]]
+
+biases2 = [-1, 2, -0.5]
+
+# Make the lists np.arrays so we can transpose them
+inputs_array = np.array(inputs)
+weights_array = np.array(weights)
+biases_array = np.array(biases)
+weights2_array = np.array(weights2)
+biases2_array = np.array(biases2)
+
+# Get the output of the first layer
+layer1_outputs = np.dot(inputs_array, weights_array.T) + biases_array
+
+# Feed the output of the first layer into the second layer with their own weights
+layer2_outputs = np.dot(layer1_outputs, weights2_array.T) + biases2_array
+
+print(layer2_outputs)
+
+# %% [markdown]
+# # 4 Layers and Batch of Data Using Numpy
+# Cascading neural network with 4 layers. Layer 1 has 4 neurons, layer 2 has 3, layer 3 has 2 and layer 4 has a single output.
+
+# %%
+import numpy as np
+
+# Batch of inputs
+inputs = [
+    [1, 2, 3, 2.5],
+    [2., 5., -1., 2],
+    [-1.5, 2.7, 3.3, -0.8]
+]
+
+# Weights are stored in a list of np.array
+# index 0 = layer 1 weights and so on
+weights = [
+    np.array([  # Layer 1
+        [0.2, 0.8, -0.5, 1],            # Neuron 1.1
+        [0.5, -0.91, 0.26, -0.5],       # Neuron 1.2
+        [-0.26, -0.27, 0.17, 0.87],     # Neuron 1.3
+        [-0.27, 0.87, 0.2, 0.8],        # Neuron 1.4
+    ]),
+    np.array([  # Layer 2
+        [0.1, 0.65, -0.24, 1.2],        # Neuron 2.1
+        [0.51, -0.21, 0.206, -0.05],    # Neuron 2.2
+        [-0.46, -0.67, 0.14, 0.37],     # Neuron 2.3
+    ]),
+    np.array([  # Layer 3
+        [0.25, 0.4, -0.2],              # Neuron 3.1
+        [0.58, -0.25, 0.26],            # Neuron 3.2
+    ]),
+    np.array([  # Layer 4
+        [0.3, 0.1],                     # Neuron 4.1
+    ])
+]
+biases = [
+    [0.5, 2, 1, -2],                    # Layer 1
+    [0.2, -0.6, 1.3,],                  # Layer 2
+    [-1, 0.5,],                         # Layer 3
+    [0.28],                             # Layer 4
+]
+
+# Iterate through each layer, get the output of the layer, use it as the input for the next layer
+layer_inputs = inputs
+layer_outputs = np.array([])
+for layer in range(len(weights)):
+    layer_weights = weights[layer]
+    layer_biases = biases[layer]
+    layer_outputs = np.dot(layer_inputs, layer_weights.T) + layer_biases
+    
+    # Update the inputs for the next layer based on the outputs of this layer
+    layer_inputs = layer_outputs
+# layer_outputs is left holding the final outputs of the network
+
+print(layer_outputs)
+
+# %% [markdown]
+# # Generating Non Linear Training Data
+# Generate 2D training data, x and y
+
+# %%
+from nnfs.datasets import spiral_data
+import numpy as np
+import nnfs
+nnfs.init()
+import matplotlib.pyplot as plt
+X, y = spiral_data(samples=100, classes=3)
+plt.scatter(X[:, 0], X[:, 1], c=y, cmap='brg')
+plt.show()
+
+# %% [markdown]
+# # Dense Layer Class
+# Given number of neurons and inputs in the layer, predict the output by assigning weights and biases.
+
+# %%
+import numpy as np
+import nnfs
+from nnfs.datasets import spiral_data
+nnfs.init()
+
+class Layer_Dense:
+    def __init__(self, n_inputs, n_neurons):
+        # Initialize the weights and biases
+        self.weights = 0.01 * np.random.randn(n_inputs, n_neurons)  # Normal distribution of weights
+        self.biases = np.zeros((1, n_neurons))
+
+    def forward(self, inputs):
+        # Calculat ethe output values from inputs, weights, and biases
+        self.output = np.dot(inputs, self.weights) + self.biases        # Weights are already transposed
+
+# Create a dataset
+X, y = spiral_data(samples=100, classes=3)
+# Create a dense layer with 2 inputs and 3 output values
+dense1 = Layer_Dense(2, 3)
+# Perform a forward pass of the dataset through the dense layer
+dense1.forward(X)
+
+# Print just the first few outputs
+print(dense1.output[:5])
+
+
+# %% [markdown]
+# # Activation Function: ReLU
+# Rectified Linear Unit
+# 
+
+# %%
+
+
+