diff --git a/pylops/utils/metrics.py b/pylops/utils/metrics.py
index e1e7a55c..6393dec2 100644
--- a/pylops/utils/metrics.py
+++ b/pylops/utils/metrics.py
@@ -5,10 +5,13 @@
     "psnr",
 ]
 
+from typing import Optional
+
 import numpy as np
+import numpy.typing as npt
 
 
-def mae(xref, xcmp):
+def mae(xref: npt.ArrayLike, xcmp: npt.ArrayLike) -> float:
     """Mean Absolute Error (MAE)
 
     Compute Mean Absolute Error between two vectors
@@ -30,7 +33,7 @@ def mae(xref, xcmp):
     return mae
 
 
-def mse(xref, xcmp):
+def mse(xref: npt.ArrayLike, xcmp: npt.ArrayLike) -> float:
     """Mean Square Error (MSE)
 
     Compute Mean Square Error between two vectors
@@ -52,7 +55,7 @@ def mse(xref, xcmp):
     return mse
 
 
-def snr(xref, xcmp):
+def snr(xref: npt.ArrayLike, xcmp: npt.ArrayLike) -> float:
     """Signal to Noise Ratio (SNR)
 
     Compute Signal to Noise Ratio between two vectors
@@ -75,7 +78,12 @@ def snr(xref, xcmp):
     return snr
 
 
-def psnr(xref, xcmp, xmax=None, xmin=0.0):
+def psnr(
+    xref: npt.ArrayLike,
+    xcmp: npt.ArrayLike,
+    xmax: Optional[float] = None,
+    xmin: Optional[float] = 0.0,
+) -> float:
     """Peak Signal to Noise Ratio (PSNR)
 
     Compute Peak Signal to Noise Ratio between two vectors