diff --git a/packages/validation/tests/test_validation.py b/packages/validation/tests/test_validation.py
index 24682e6c..eaf77da2 100644
--- a/packages/validation/tests/test_validation.py
+++ b/packages/validation/tests/test_validation.py
@@ -1,9 +1,92 @@
 from pathlib import Path
 
+import pytest
+
+from validation import main as validation_main
 from validation import validate_eicr
 
 
+class FakeAssert:
+    local_name = "failed-assert"
+
+    def get_attribute_value(self, attribute: str) -> str:
+        values = {
+            "id": "ttc-labTestNameOrdered-noCode",
+            "location": "/ClinicalDocument/component/structuredBody/component/section/entry/observation",
+        }
+
+        return values[attribute]
+
+
+class FakeRoot:
+    def __init__(self) -> None:
+        """Represents the root of the SVRL output, which contains failed-assert children."""
+        self.children = [FakeAssert()]
+
+
+class FakeExecutable:
+    def apply_templates_returning_value(self, xdm_value: str) -> list[list[FakeRoot]]:
+        """Simulates applying the XSLT stylesheet to the XML document and returning SVRL output."""
+        return [[FakeRoot()]]
+
+
+class FakeXsltProcessor:
+    def __init__(self) -> None:
+        """Simulates the XSLT processor, recording transformations applied."""
+        self.transforms: list[tuple[str, str, str]] = []
+
+    def transform_to_file(self, source_file: str, stylesheet_file: str, output_file: str) -> None:
+        """Simulates transforming an XML file with an XSLT stylesheet and writing to an output file."""
+        self.transforms.append((source_file, stylesheet_file, output_file))
+        Path(output_file).write_text("<generated />")
+
+    def compile_stylesheet(self, stylesheet_file: str) -> FakeExecutable:
+        """Simulates compiling an XSLT stylesheet into an executable."""
+        return FakeExecutable()
+
+
+class FakeSaxonProcessor:
+    version = "Fake Saxon/C"
+    """Simulates the Saxon/C processor."""
+
+    def __init__(self, license: bool) -> None:
+        """Initializes the Saxon/C processor with a license flag and an XSLT processor."""
+        self.license = license
+        self.xslt_processor = FakeXsltProcessor()
+
+    def __enter__(self) -> "FakeSaxonProcessor":
+        """Enters the context manager, returning itself to be used for transformations."""
+        return self
+
+    def __exit__(self, exc_type: object, exc_value: object, traceback: object) -> None:
+        """Exits the context manager, performing any necessary cleanup (none in this fake implementation)."""
+        return
+
+    def new_xslt30_processor(self) -> FakeXsltProcessor:
+        """Returns the XSLT processor to be used for transformations."""
+        return self.xslt_processor
+
+    def parse_xml(self, xml_text: str | None) -> str:
+        """Simulates parsing an XML string into an XDM node, which in this fake implementation is just the string itself."""
+        return xml_text or ""
+
+
+class BrokenSaxonProcessor:
+    def __init__(self, license: bool) -> None:
+        """Simulates a Saxon/C processor that raises an error when used, to test error handling in the validation function."""
+        self.license = license
+
+    def __enter__(self) -> "BrokenSaxonProcessor":
+        """Raises a RuntimeError to simulate a failure when entering the context manager."""
+        raise RuntimeError("validator failed")
+
+    def __exit__(self, exc_type: object, exc_value: object, traceback: object) -> None:
+        """Exits the context manager, which in this case is not reached due to the error raised in __enter__."""
+        return
+
+
 def test_validation():
+    """Tests that the validate_eicr function correctly processes an eICR and returns expected validation results."""
     with Path.open("packages/validation/tests/assets/test_eicr.xml") as f:
         eicr = f.read()
     results = validate_eicr(eicr)
@@ -17,8 +100,82 @@ def test_validation():
 
 
 def test_validation_no_errors():
+    """Tests that the validate_eicr function returns an empty list when there are no validation errors."""
     with Path.open("e2e/snapshots/test_e2e/test_upload_and_process/augmented_eicr.xml") as f:
         eicr = f.read()
     results = validate_eicr(eicr)
 
     assert results == []
+
+
+def test_validation_redoes_all_steps(monkeypatch: pytest.MonkeyPatch, tmp_path: Path):
+    """Tests that the validate_eicr function redoes all steps of the validation process when redo_all_steps is True, and that it returns expected validation results."""
+    stage1_output = tmp_path / "stage1.sch.tmp"
+    stage2_output = tmp_path / "stage2.sch.tmp"
+    validator_output = tmp_path / "validator.xsl.tmp"
+
+    stage1_output.write_text("old stage 1")
+    stage2_output.write_text("old stage 2")
+    validator_output.write_text("old validator")
+
+    monkeypatch.setattr(validation_main, "STAGE1_OUTPUT", stage1_output)
+    monkeypatch.setattr(validation_main, "STAGE2_OUTPUT", stage2_output)
+    monkeypatch.setattr(validation_main, "VALIDATOR_OUTPUT", validator_output)
+    monkeypatch.setattr(validation_main, "APHL_SCHEMATRON", tmp_path / "schema.sch")
+    monkeypatch.setattr(validation_main, "XSLT_INCLUDE", tmp_path / "include.xsl")
+    monkeypatch.setattr(validation_main, "XSLT_EXPAND", tmp_path / "expand.xsl")
+    monkeypatch.setattr(validation_main, "XSLT_COMPILE", tmp_path / "compile.xsl")
+    monkeypatch.setattr(validation_main, "PySaxonProcessor", FakeSaxonProcessor)
+
+    results = validate_eicr("<ClinicalDocument />", redo_all_steps=True)
+
+    assert results == [
+        {
+            "error_id": "ttc-labTestNameOrdered-noCode",
+            "location": "/ClinicalDocument/component/structuredBody/component/section/entry/observation",
+        }
+    ]
+    assert stage1_output.read_text() == "<generated />"
+    assert stage2_output.read_text() == "<generated />"
+    assert validator_output.read_text() == "<generated />"
+
+
+def test_validation_uses_existing_generated_files(monkeypatch: pytest.MonkeyPatch, tmp_path: Path):
+    """Tests that the validate_eicr function uses existing generated files for steps 1-3 of the validation process when redo_all_steps is False, and that it returns expected validation results."""
+    stage1_output = tmp_path / "stage1.sch.tmp"
+    stage2_output = tmp_path / "stage2.sch.tmp"
+    validator_output = tmp_path / "validator.xsl.tmp"
+
+    stage1_output.write_text("existing stage 1")
+    stage2_output.write_text("existing stage 2")
+    validator_output.write_text("existing validator")
+
+    monkeypatch.setattr(validation_main, "STAGE1_OUTPUT", stage1_output)
+    monkeypatch.setattr(validation_main, "STAGE2_OUTPUT", stage2_output)
+    monkeypatch.setattr(validation_main, "VALIDATOR_OUTPUT", validator_output)
+    monkeypatch.setattr(validation_main, "PySaxonProcessor", FakeSaxonProcessor)
+
+    results = validate_eicr("<ClinicalDocument />")
+
+    assert results == [
+        {
+            "error_id": "ttc-labTestNameOrdered-noCode",
+            "location": "/ClinicalDocument/component/structuredBody/component/section/entry/observation",
+        }
+    ]
+    assert stage1_output.read_text() == "existing stage 1"
+    assert stage2_output.read_text() == "existing stage 2"
+    assert validator_output.read_text() == "existing validator"
+
+
+def test_validation_returns_empty_list_when_validator_errors(
+    monkeypatch: pytest.MonkeyPatch,
+    caplog: pytest.LogCaptureFixture,
+):
+    """Tests that the validate_eicr function returns an empty list and logs an error when the validator fails."""
+    monkeypatch.setattr(validation_main, "PySaxonProcessor", BrokenSaxonProcessor)
+
+    results = validate_eicr("<ClinicalDocument />")
+
+    assert results == []
+    assert "An error occurred during validation: validator failed" in caplog.text