diff --git a/tests/trainer/test_config_validator.py b/tests/trainer/test_config_validator.py
index 1ab97304f2338b..d11bdf1d64e5a5 100755
--- a/tests/trainer/test_config_validator.py
+++ b/tests/trainer/test_config_validator.py
@@ -92,3 +92,23 @@ def test_test_loop_config(tmpdir):
         model = EvalModelTemplate(**hparams)
         model.test_step = None
         trainer.test(model, test_dataloaders=model.dataloader(train=False))
+
+
+def test_validation_loop_config(tmpdir):
+    """"
+    When either validation loop or validation data are missing
+    """
+    hparams = EvalModelTemplate.get_default_hparams()
+    trainer = Trainer(default_root_dir=tmpdir, max_epochs=1)
+
+    # has val loop but no val data
+    with pytest.warns(UserWarning):
+        model = EvalModelTemplate(**hparams)
+        model.val_dataloader = None
+        trainer.validate(model)
+
+    # has val data but no val loop
+    with pytest.warns(UserWarning):
+        model = EvalModelTemplate(**hparams)
+        model.validation_step = None
+        trainer.validate(model, val_dataloaders=model.dataloader(train=False))
diff --git a/tests/trainer/test_dataloaders.py b/tests/trainer/test_dataloaders.py
index 1ca34101a9141a..9d505c246b6f53 100644
--- a/tests/trainer/test_dataloaders.py
+++ b/tests/trainer/test_dataloaders.py
@@ -170,6 +170,45 @@ def test_step(self, batch, batch_idx, *args, **kwargs):
     trainer.test(ckpt_path=ckpt_path)
 
 
+@pytest.mark.parametrize('ckpt_path', [None, 'best', 'specific'])
+def test_multiple_validate_dataloader(tmpdir, ckpt_path):
+    """Verify multiple val_dataloaders."""
+
+    model_template = EvalModelTemplate()
+
+    class MultipleValDataloaderModel(EvalModelTemplate):
+        def val_dataloader(self):
+            return model_template.val_dataloader__multiple()
+
+        def validation_step(self, batch, batch_idx, *args, **kwargs):
+            return model_template.validation_step__multiple_dataloaders(batch, batch_idx, *args, **kwargs)
+
+    model = MultipleValDataloaderModel()
+
+    # fit model
+    trainer = Trainer(
+        default_root_dir=tmpdir,
+        max_epochs=1,
+        limit_val_batches=0.1,
+        limit_train_batches=0.2,
+    )
+    trainer.fit(model)
+    if ckpt_path == 'specific':
+        ckpt_path = trainer.checkpoint_callback.best_model_path
+    trainer.validate(ckpt_path=ckpt_path)
+
+    # verify there are 2 test loaders
+    assert len(trainer.val_dataloaders) == 2, \
+        'Multiple val_dataloaders not initiated properly'
+
+    # make sure predictions are good for each test set
+    for dataloader in trainer.val_dataloaders:
+        tpipes.run_prediction(dataloader, trainer.model)
+
+    # run the test method
+    trainer.validate(ckpt_path=ckpt_path)
+
+
 def test_train_dataloader_passed_to_fit(tmpdir):
     """Verify that train dataloader can be passed to fit """
 
diff --git a/tests/trainer/test_optimizers.py b/tests/trainer/test_optimizers.py
index c28e626f2eec08..52dc14eb979948 100644
--- a/tests/trainer/test_optimizers.py
+++ b/tests/trainer/test_optimizers.py
@@ -335,6 +335,24 @@ def test_init_optimizers_during_testing(tmpdir):
     assert len(trainer.optimizer_frequencies) == 0
 
 
+def test_init_optimizers_during_validation(tmpdir):
+    """
+    Test that optimizers is an empty list during validation.
+    """
+    model = EvalModelTemplate()
+    model.configure_optimizers = model.configure_optimizers__multiple_schedulers
+
+    trainer = Trainer(
+        default_root_dir=tmpdir,
+        limit_test_batches=10
+    )
+    trainer.validate(model, ckpt_path=None)
+
+    assert len(trainer.lr_schedulers) == 0
+    assert len(trainer.optimizers) == 0
+    assert len(trainer.optimizer_frequencies) == 0
+
+
 def test_multiple_optimizers_callbacks(tmpdir):
     """
     Tests that multiple optimizers can be used with callbacks
diff --git a/tests/trainer/test_states.py b/tests/trainer/test_states.py
index 0244f654227a2f..f6de8deb9e0747 100644
--- a/tests/trainer/test_states.py
+++ b/tests/trainer/test_states.py
@@ -191,6 +191,40 @@ def test_finished_state_after_test(tmpdir):
     assert trainer.state == TrainerState.FINISHED
 
 
+def test_running_state_during_validation(tmpdir):
+    """ Tests that state is set to RUNNING during test """
+
+    hparams = EvalModelTemplate.get_default_hparams()
+    model = EvalModelTemplate(**hparams)
+
+    snapshot_callback = StateSnapshotCallback(snapshot_method='on_validation_batch_start')
+
+    trainer = Trainer(
+        callbacks=[snapshot_callback],
+        default_root_dir=tmpdir,
+        fast_dev_run=True,
+    )
+
+    trainer.validate(model)
+
+    assert snapshot_callback.trainer_state == TrainerState.RUNNING
+
+
+def test_finished_state_after_validation(tmpdir):
+    """ Tests that state is FINISHED after fit """
+    hparams = EvalModelTemplate.get_default_hparams()
+    model = EvalModelTemplate(**hparams)
+
+    trainer = Trainer(
+        default_root_dir=tmpdir,
+        fast_dev_run=True,
+    )
+
+    trainer.validate(model)
+
+    assert trainer.state == TrainerState.FINISHED
+
+
 @pytest.mark.parametrize("extra_params", [
     pytest.param(dict(fast_dev_run=True), id='Fast-Run'),
     pytest.param(dict(max_steps=1), id='Single-Step'),
diff --git a/tests/trainer/test_trainer.py b/tests/trainer/test_trainer.py
index 1fbfbb7eb137ae..94c26a9c23462d 100644
--- a/tests/trainer/test_trainer.py
+++ b/tests/trainer/test_trainer.py
@@ -747,6 +747,47 @@ def test_test_checkpoint_path(tmpdir, ckpt_path, save_top_k):
             assert trainer.tested_ckpt_path == ckpt_path
 
 
+@pytest.mark.parametrize("ckpt_path", [None, "best", "specific"])
+@pytest.mark.parametrize("save_top_k", [-1, 0, 1, 2])
+def test_validate_checkpoint_path(tmpdir, ckpt_path, save_top_k):
+    hparams = EvalModelTemplate.get_default_hparams()
+
+    model = EvalModelTemplate(**hparams)
+    trainer = Trainer(
+        max_epochs=2,
+        progress_bar_refresh_rate=0,
+        default_root_dir=tmpdir,
+        checkpoint_callback=ModelCheckpoint(monitor="early_stop_on", save_top_k=save_top_k),
+    )
+    trainer.fit(model)
+    if ckpt_path == "best":
+        # ckpt_path is 'best', meaning we load the best weights
+        if save_top_k == 0:
+            with pytest.raises(MisconfigurationException, match=".*is not configured to save the best.*"):
+                trainer.validate(ckpt_path=ckpt_path)
+        else:
+            trainer.validate(ckpt_path=ckpt_path)
+            assert trainer.tested_ckpt_path == trainer.checkpoint_callback.best_model_path
+    elif ckpt_path is None:
+        # ckpt_path is None, meaning we don't load any checkpoints and
+        # use the weights from the end of training
+        trainer.validate(ckpt_path=ckpt_path)
+        assert trainer.tested_ckpt_path is None
+    else:
+        # specific checkpoint, pick one from saved ones
+        if save_top_k == 0:
+            with pytest.raises(FileNotFoundError):
+                trainer.validate(ckpt_path="random.ckpt")
+        else:
+            ckpt_path = str(
+                list((Path(tmpdir) / f"lightning_logs/version_{trainer.logger.version}/checkpoints").iterdir())[
+                    0
+                ].absolute()
+            )
+            trainer.validate(ckpt_path=ckpt_path)
+            assert trainer.tested_ckpt_path == ckpt_path
+
+
 def test_disabled_training(tmpdir):
     """Verify that `limit_train_batches=0` disables the training loop unless `fast_dev_run=True`."""
 
@@ -1448,6 +1489,10 @@ def setup(self, model, stage):
     assert trainer.stage == "test"
     assert trainer.get_model().stage == "test"
 
+    trainer.validate(ckpt_path=None)
+    assert trainer.stage == "validation"
+    assert trainer.get_model().stage == "validation"
+
 
 @pytest.mark.parametrize(
     "train_batches, max_steps, log_interval",
diff --git a/tests/trainer/test_trainer_validate_loop.py b/tests/trainer/test_trainer_validate_loop.py
new file mode 100644
index 00000000000000..a2205a4b50dc28
--- /dev/null
+++ b/tests/trainer/test_trainer_validate_loop.py
@@ -0,0 +1,76 @@
+# Copyright The PyTorch Lightning team.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+import pytest
+import torch
+
+import pytorch_lightning as pl
+import tests.base.develop_utils as tutils
+from tests.base import EvalModelTemplate
+
+
+@pytest.mark.skipif(torch.cuda.device_count() < 2, reason="test requires multi-GPU machine")
+def test_single_gpu_validate(tmpdir):
+    tutils.set_random_master_port()
+
+    model = EvalModelTemplate()
+    trainer = pl.Trainer(
+        default_root_dir=tmpdir,
+        max_epochs=2,
+        limit_train_batches=10,
+        limit_val_batches=10,
+        gpus=[0],
+    )
+    trainer.fit(model)
+    assert 'ckpt' in trainer.checkpoint_callback.best_model_path
+    results = trainer.validate()
+    assert 'val_acc' in results[0]
+
+    old_weights = model.c_d1.weight.clone().detach().cpu()
+
+    results = trainer.validate(model)
+    assert 'val_acc' in results[0]
+
+    # make sure weights didn't change
+    new_weights = model.c_d1.weight.clone().detach().cpu()
+
+    assert torch.all(torch.eq(old_weights, new_weights))
+
+
+@pytest.mark.skipif(torch.cuda.device_count() < 2, reason="test requires multi-GPU machine")
+def test_ddp_spawn_validate(tmpdir):
+    tutils.set_random_master_port()
+
+    model = EvalModelTemplate()
+    trainer = pl.Trainer(
+        default_root_dir=tmpdir,
+        max_epochs=2,
+        limit_train_batches=10,
+        limit_val_batches=10,
+        gpus=[0, 1],
+        distributed_backend='ddp_spawn',
+    )
+    trainer.fit(model)
+    assert 'ckpt' in trainer.checkpoint_callback.best_model_path
+    results = trainer.validate()
+    assert 'val_acc' in results[0]
+
+    old_weights = model.c_d1.weight.clone().detach().cpu()
+
+    results = trainer.validate(model)
+    assert 'val_acc' in results[0]
+
+    # make sure weights didn't change
+    new_weights = model.c_d1.weight.clone().detach().cpu()
+
+    assert torch.all(torch.eq(old_weights, new_weights))