diff --git a/seismostats/analysis/estimate_a.py b/seismostats/analysis/estimate_a.py
new file mode 100644
index 0000000..71bde0c
--- /dev/null
+++ b/seismostats/analysis/estimate_a.py
@@ -0,0 +1,64 @@
+"""This module contains functions for the estimation of a-value.
+"""
+
+import numpy as np
+import datetime as dt
+import warnings
+
+
+def estimate_a(magnitudes: np.ndarray,
+               mc: float | None = None,
+               m0: float | None = None,
+               b_value: float | None = None,
+               delta_T: dt.timedelta | None = None,
+               T: np.ndarray | None = None,
+               ) -> float:
+    """Estimate the a-value of the Gutenberg-Richter (GR) law.
+    N  T/ delta_T = 10^(a - b * (m0 - mc)) (1)
+    where N is the number of events with magnitude greater than m0, T is the
+    time interval the magnitudes are observed, delta_T is the time interval
+    for the estimation of the a-value.
+    The original GR law was without the factor T/ delta_T - however, in many
+    contexts this has proven useful so that a-values for the same region can be
+    compared.
+
+    Args:
+        magnitudes: Magnitude sample
+        mc:         Completeness magnitude. If None, the lowest magnitude is
+                used as completeness magnitude.
+        m0:         Magnitude for which the a-value is estimated. If None, the
+                a-value is estimated at mc.
+        b_value:    b-value of the Gutenberg-Richter distribution
+        delta_T:    Time interval for the estimation of the a-value. If None,
+            delta_T is set to the whole interval of the times. Often, delta_T
+            is set to 1 year.
+        T:          Time interval the magnitudes are observed. Only needed if 
+            delta_T is not None.
+
+    Returns:
+        a: a-value of the Gutenberg-Richter distribution
+    """
+
+    if mc is None:
+        mc = magnitudes.min()
+
+    if m0 is None:
+        m0 = mc
+    elif m0 != mc:
+        if b_value is None:
+            raise ValueError("b_value must be provided if m0 not equal to mc.")
+
+    if magnitudes.min() < mc:
+        warnings.warn(
+            "Completeness magnitude is higher than the lowest magnitude."
+            "Cutting the magnitudes to the completeness magnitude.")
+        magnitudes = magnitudes[magnitudes >= mc]
+
+    a = np.log10(len(magnitudes)) - b_value * (m0 - mc)
+
+    if delta_T is not None:
+        if T is None:
+            raise ValueError("T must be provided if delta_T is given.")
+        a = a + np.log(delta_T / T)
+
+    return a