diff --git a/espaloma/__init__.py b/espaloma/__init__.py
index 42d274bf..4b934d1f 100644
--- a/espaloma/__init__.py
+++ b/espaloma/__init__.py
@@ -13,6 +13,8 @@
 import espaloma.nn
 import espaloma.graphs
 import espaloma.mm
+import espaloma.app
+
 
 from espaloma.mm.geometry import *
 
diff --git a/espaloma/app/__init__.py b/espaloma/app/__init__.py
new file mode 100644
index 00000000..e1d0740c
--- /dev/null
+++ b/espaloma/app/__init__.py
@@ -0,0 +1,3 @@
+import espaloma
+import espaloma.app
+import espaloma.app.experiment
diff --git a/espaloma/app/experiment.py b/espaloma/app/experiment.py
new file mode 100644
index 00000000..bb9c98aa
--- /dev/null
+++ b/espaloma/app/experiment.py
@@ -0,0 +1,214 @@
+# =============================================================================
+# IMPORTS
+# =============================================================================
+import espaloma as esp
+import abc
+import torch
+import copy
+
+# =============================================================================
+# MODULE CLASSES
+# =============================================================================
+class Experiment(abc.ABC):
+    """ Base class for espaloma experiment.
+
+    """
+    def __init__(self):
+        super(Experiment, self).__init__()
+
+class Train(Experiment):
+    """ Train a model for a while.
+    """
+    def __init__(
+            self,
+            net,
+            data,
+            metrics=[esp.metrics.TypingCrossEntropy],
+            optimizer=lambda net: torch.optim.Adam(net.parameters(), 1e-3),
+            n_epochs=100,
+            record_interval=1,
+        ):
+        super(Train, self).__init__()
+
+        # bookkeeping
+        self.net = net
+        self.data = data
+        self.metrics = metrics
+        self.n_epochs = n_epochs
+        self.record_interval = record_interval
+        self.states = {}
+
+        # make optimizer
+        if callable(optimizer):
+            self.optimizer = optimizer(net)
+        else:
+            self.optimizer = optimizer
+
+        # compose loss function
+        def loss(g):
+            _loss = 0.
+            for metric in self.metrics:
+                _loss += metric(g)
+            return _loss
+
+        self.loss = loss
+
+    def train_once(self):
+        """ Train the model for one batch.
+
+        """
+        for g in self.data: # TODO: does this have to be a single g?
+
+            def closure():
+                self.optimizer.zero_grad()
+                loss = self.loss(g)
+                loss.backward()
+                return loss
+
+            self.optimizer.step()
+
+    def train(self):
+        """ Train the model for multiple steps and
+        record the weights once every `record_interval`
+
+        """
+        for epoch_idx in range(int(self.n_epochs)):
+            self.train_once()
+
+            # record when `record_interval` is hit
+            if epoch_idx % self.record_interval == 0:
+                self.states[epoch_idx] = copy.deepcopy(self.net.state_dict())
+
+        # record final state
+        self.states['final'] = copy.deepcopy(self.net.state_dict())
+
+        return self.net
+
+
+class Test(Experiment):
+    """ Run sequences of tests on a trained model.
+
+    """
+    def __init__(
+            self,
+            net,
+            data,
+            states,
+            metrics=[esp.metrics.TypingCrossEntropy],
+            sampler=None):
+        # bookkeeping
+        self.net = net
+        self.data = data
+        self.states = states
+        self.metrics = metrics
+        self.sampler = sampler
+
+    def test(self):
+        """ Run test.
+
+        """
+        # loop through the metrics
+        for metric in metrics:
+            results[metric.__name__] = {}
+
+        # make it just one giant graph
+        g = data[0:-1]
+
+        for state_name, state in self.states.items(): # loop through states
+            # load the state dict
+            self.net.load_state_dict(state)
+
+            # loop through the metrics
+            results[metric.__name__][state_name] = metric(
+                self.net,
+                g,
+                sampler=self.sampler).detach().cpu().numpy()
+
+        # point this to self
+        self.results = results
+        return dict(results)
+
+class TrainAndTest(Experiment):
+    """ Train a model and then test it.
+
+    """
+    def __init__(
+        self,
+        net,
+        ds_tr,
+        ds_te,
+        metrics_tr=[esp.metrics.TypingCrossEntropy],
+        metrics_te=[esp.metrics.TypingCrossEntropy],
+        optimizer=lambda net: torch.optim.Adam(net.parameters(), 1e-3),
+        n_epochs=100,
+        record_interval=1
+    ):
+
+        # bookkeeping
+        self.net = net
+        self.ds_tr = ds_tr
+        self.ds_te = ds_te
+        self.optimizer = optimizer
+        self.n_epochs = n_epochs
+
+    def __str__(self):
+        _str = ''
+        _str += '# model'
+        _str += '\n'
+        _str += str(self.net)
+        _str += '\n'
+        if hasattr(self.net, 'noise_model'):
+            _str += '# noise model'
+            _str += '\n'
+            _str += str(self.net.noise_model)
+            _str += '\n'
+        _str += '# optimizer'
+        _str += '\n'
+        _str += str(self.optimizer)
+        _str += '\n'
+        _str += '# n_epochs'
+        _str += '\n'
+        _str += str(self.n_epochs)
+        _str += '\n'
+        return _str
+
+    def run(self):
+        """ Run train and test.
+
+        """
+        train = Train(
+            net=self.net,
+            data=self.ds_tr,
+            optimizer=self.optimizer,
+            n_epochs=self.n_epochs
+        )
+
+        train.train()
+
+        self.states = train.states
+
+        test = Test(
+            net=self.net,
+            data=self.ds_te,
+            metrics=self.metrics,
+            states=self.states,
+            sampler=self.sampler
+        )
+
+        test.test()
+
+        self.results_te = test.results
+
+        test = Test(
+            net=self.net,
+            data=self.ds_tr,
+            metrics=self.metrics,
+            states=self.states,
+            sampler=self.sampler
+        )
+
+        test.test()
+
+        self.results_tr = test.results
+
+        return{'test': self.results_te, 'train': self.results_tr}
diff --git a/espaloma/app/tests/test_experiment.py b/espaloma/app/tests/test_experiment.py
new file mode 100644
index 00000000..92057e0c
--- /dev/null
+++ b/espaloma/app/tests/test_experiment.py
@@ -0,0 +1,83 @@
+import pytest
+import torch
+
+def test_import():
+    import espaloma as esp
+    esp.app.experiment
+
+
+@pytest.fixture
+def data():
+    import espaloma as esp
+
+    esol = esp.data.esol(first=20)
+
+    # do some typing
+    typing = esp.graphs.legacy_force_field.LegacyForceField('gaff-1.81')
+    esol.apply(typing, in_place=True) # this modify the original data
+
+    return esol
+
+@pytest.fixture
+def net():
+    import espaloma as esp
+
+    # define a layer
+    layer = esp.nn.layers.dgl_legacy.gn('GraphConv')
+
+    # define a representation
+    representation = esp.nn.Sequential(
+            layer,
+            [32, 'tanh', 32, 'tanh', 32, 'tanh'])
+
+    # define a readout
+    readout = esp.nn.readout.node_typing.NodeTyping(
+            in_features=32,
+            n_classes=100) # not too many elements here I think?
+
+    net = torch.nn.Sequential(
+        representation,
+        readout)
+
+    return net
+
+
+def test_data_and_net(data, net):
+    data
+    net
+
+
+@pytest.fixture
+def train(data, net):
+    import espaloma as esp
+    train = esp.app.experiment.Train(
+        net=net,
+        data=data,
+        n_epochs=1,
+        metrics=[esp.metrics.GraphMetric(
+            base_metric=torch.nn.CrossEntropyLoss(),
+            between=['nn_typing', 'legacy_typing'])])
+
+    return train
+
+def test_train(train):
+    train.train()
+
+def test_test(train, net, data):
+    import espaloma as esp
+    train.train()
+    test = esp.app.experiment.Test(
+        net=net,
+        data=data,
+        states=train.states)
+
+
+def test_train_and_test(net, data):
+    import espaloma as esp
+
+    train_and_test = esp.app.experiment.TrainAndTest(
+        net=net,
+        n_epochs=1,
+        ds_tr=data,
+        ds_te=data
+    )
diff --git a/espaloma/graphs/legacy_force_field.py b/espaloma/graphs/legacy_force_field.py
index 67884957..d1620a6e 100644
--- a/espaloma/graphs/legacy_force_field.py
+++ b/espaloma/graphs/legacy_force_field.py
@@ -42,7 +42,7 @@ def __init__(self, forcefield="gaff-1.81"):
     @staticmethod
     def _convert_to_off(mol):
         import openforcefield
-        
+
         if isinstance(mol, esp.Graph):
             return mol.mol
 
@@ -149,7 +149,7 @@ def _type_gaff(self, mol, g=None):
         return g
 
     def typing(self, mol, g=None):
-        """ Type a molecular graph. 
+        """ Type a molecular graph.
 
         """
         if "gaff" in self.forcefield:
diff --git a/espaloma/metrics.py b/espaloma/metrics.py
index 08ea3e6e..bac16d04 100644
--- a/espaloma/metrics.py
+++ b/espaloma/metrics.py
@@ -39,24 +39,24 @@ def r2(y, y_hat):
 # =============================================================================
 # MODULE CLASSES
 # =============================================================================
-class Loss(torch.nn.modules.loss._Loss):
+class Metric(torch.nn.modules.loss._Loss):
     """ Base function for loss.
 
     """
     def __init__(self, size_average=None, reduce=None, reduction='mean'):
-        super(Loss, self).__init__(size_average, reduce, reduction)
+        super(Metric, self).__init__(size_average, reduce, reduction)
 
     @abc.abstractmethod
     def forward(self, *args, **kwargs):
         raise NotImplementedError
 
-class GraphLoss(Loss):
+class GraphMetric(Metric):
     """ Loss between nodes attributes of graph or graphs.
 
     """
-    def __init__(self, base_loss, between, *args, **kwargs):
-        super(GraphLoss, self).__init__(*args, **kwargs)
-        
+    def __init__(self, base_metric, between, *args, **kwargs):
+        super(GraphMetric, self).__init__(*args, **kwargs)
+
         # between could be tuple of two strings or two functions
         assert len(between) == 2
 
@@ -64,7 +64,7 @@ def __init__(self, base_loss, between, *args, **kwargs):
                 self._translation(between[0]),
                 self._translation(between[1]))
 
-        self.base_loss = base_loss
+        self.base_metric = base_metric
 
     @staticmethod
     def _translation(string):
@@ -73,7 +73,7 @@ def _translation(string):
             'legacy_typing': lambda g: g.ndata['legacy_typing']
         }[string]
 
-    
+
     def forward(self, g_input, g_target=None):
         """ Forward function of loss.
 
@@ -88,9 +88,12 @@ def forward(self, g_input, g_target=None):
         # compute loss using base loss
         # NOTE:
         # use keyward argument here since torch is bad with the order with args
-        return self.base_loss.forward(
+        return self.base_metric.forward(
                 input=input_fn(g_input),
                 target=target_fn(g_target))
 
-
-
+class TypingCrossEntropy(Metric):
+    def __init__(self):
+        super(TypingCrossEntropy).__init__(
+            base_metric=torch.nn.CrossEntropyLoss(),
+            between=['nn_typing', 'legacy_typing'])