diff --git a/requirements.txt b/requirements.txt
index af2f050..a649b51 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -7,6 +7,7 @@ pytest==7.4.2
 pytest-cov==4.1.0
 pytest-emoji==0.2.0
 pytest-md==0.2.0
+pytest-mock==3.11.1
 pytest-xdist==3.3.1
 tqdm==4.66.1
 typing-extensions==4.8.0
diff --git a/test/conftest.py b/test/conftest.py
index 500bb5b..6cbac30 100644
--- a/test/conftest.py
+++ b/test/conftest.py
@@ -7,6 +7,7 @@
 import pytest
 
 from src.click_opt.cli_options import CliOptions
+from src.esxport import EsXport
 
 
 @pytest.fixture()
@@ -48,3 +49,9 @@ def mock_es_client() -> Mock:
         },
     }
     return mock_client
+
+
+@pytest.fixture()
+def esxport_obj(cli_options: CliOptions, mock_es_client: Mock) -> EsXport:
+    """Mocked EsXport class."""
+    return EsXport(cli_options, mock_es_client)
diff --git a/test/esxport/_validate_fields_test.py b/test/esxport/_validate_fields_test.py
new file mode 100644
index 0000000..7704ff9
--- /dev/null
+++ b/test/esxport/_validate_fields_test.py
@@ -0,0 +1,101 @@
+"""Field Validator test cases."""
+from unittest.mock import Mock
+
+import pytest
+from typing_extensions import Self
+
+from src.esxport import EsXport
+from src.exceptions import FieldNotFoundError
+
+
+class TestValidateFields:
+    """Test that all expected fields exist in all indices."""
+
+    def test_all_expected_fields_exist_in_all_indices(self: Self, mocker: Mock, esxport_obj: EsXport) -> None:
+        """Test that all expected fields exist in all indices, me hearties!."""
+        # Mock the get_mapping method of ElasticsearchClient to return a mapping with all expected fields
+        mocker.patch.object(
+            esxport_obj.es_client,
+            "get_mapping",
+            return_value={
+                "index1": {
+                    "mappings": {
+                        "properties": ["field1", "field2", "field3"],
+                    },
+                },
+                "index2": {
+                    "mappings": {
+                        "properties": ["field1", "field2", "field3"],
+                    },
+                },
+            },
+        )
+
+        esxport_obj._validate_fields()
+
+    def test_all_expected_fields_exist_in_some_indices(self: Self, mocker: Mock, esxport_obj: EsXport) -> None:
+        """Ahoy!.Test that all expected fields exist in some indices, me mateys!."""
+        # Mock the get_mapping method of ElasticsearchClient to return a mapping with some expected fields
+        mocker.patch.object(
+            esxport_obj.es_client,
+            "get_mapping",
+            return_value={
+                "index1": {
+                    "mappings": {
+                        "properties": ["aaa", "bbb"],
+                    },
+                },
+                "index2": {
+                    "mappings": {
+                        "properties": ["cccc", "dddd"],
+                    },
+                },
+            },
+        )
+
+        with pytest.raises(FieldNotFoundError):
+            esxport_obj._validate_fields()
+
+    def test_all_expected_fields_exist_in_one_index(self: Self, mocker: Mock, esxport_obj: EsXport) -> None:
+        """Test that all expected fields exist in one index, me hearties!."""
+        # Mock the get_mapping method of ElasticsearchClient to return a mapping with all expected fields
+        mocker.patch.object(
+            esxport_obj.es_client,
+            "get_mapping",
+            return_value={
+                "index1": {
+                    "mappings": {
+                        "properties": ["field1", "field2", "field3"],
+                    },
+                },
+            },
+        )
+
+        esxport_obj.opts.index_prefixes = ["index1"]
+        esxport_obj.opts.fields = ["field1", "field2", "field3"]
+        esxport_obj._validate_fields()
+
+    def test_sort_param_are_checked(self: Self, mocker: Mock, esxport_obj: EsXport) -> None:
+        """Test that all expected fields exist in one index, me hearties!."""
+        # Mock the get_mapping method of ElasticsearchClient to return a mapping with all expected fields
+        mocker.patch.object(
+            esxport_obj.es_client,
+            "get_mapping",
+            return_value={
+                "index1": {
+                    "mappings": {
+                        "properties": ["field1", "field2", "field3"],
+                    },
+                },
+            },
+        )
+
+        esxport_obj.opts.index_prefixes = ["index1"]
+        esxport_obj.opts.sort = [{"abc": "desc"}, {"def": "desc"}]
+
+        with pytest.raises(FieldNotFoundError):
+            esxport_obj._validate_fields()
+
+        esxport_obj.opts.sort = [{"field1": "asc"}, {"field2": "desc"}]
+
+        esxport_obj._validate_fields()