diff --git a/.gitattributes b/.gitattributes
index d965232..f8870b6 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -131,6 +131,7 @@ invesalius/gui/widgets/gradient.py -text
 invesalius/gui/widgets/listctrl.py -text
 invesalius/gui/widgets/slice_menu.py -text
 invesalius/invesalius.py -text
+invesalius/mode.py -text
 invesalius/presets.py -text
 invesalius/project.py -text
 invesalius/reader/__init__.py -text
@@ -138,6 +139,7 @@ invesalius/reader/analyze_reader.py -text
 invesalius/reader/dicom.py -text
 invesalius/reader/dicom_grouper.py -text
 invesalius/reader/dicom_reader.py -text
+invesalius/session.py -text
 invesalius/utils.py -text
 presets/raycasting/Airways[!!-~]II.plist -text
 presets/raycasting/Airways.plist -text
diff --git a/invesalius/mode.py b/invesalius/mode.py
new file mode 100644
index 0000000..21bc73b
--- /dev/null
+++ b/invesalius/mode.py
@@ -0,0 +1,110 @@
+# mode.py
+# to be instanced inside Controller (control.py)
+
+
+# TODO: Add to constants.py:
+#--------------------
+# SLICE MODE
+SLICE_STATE_DEFAULT = 0
+SLICE_STATE_EDITOR = 1
+SLICE_STATE_WL = 2
+SLICE_STATE_SPIN = 3
+SLICE_STATE_ZOOM = 4
+SLICE_STATE_ZOOM_SL = 5
+# IMPORTANT: When adding a new state, remember o insert it into LEVEL
+# dictionary
+
+
+# RULE:
+# default is the only level 0
+# states controlled somehow by taskmenu are level 1
+# states controlled by toolbar are level 2
+LEVEL = {SLICE_STATE_DEFAULT: 0,
+         SLICE_STATE_EDITOR: 1,
+         SLICE_STATE_WL: 2,
+         SLICE_STATE_SPIN: 2,
+         SLICE_STATE_ZOOM: 2,
+         SLICE_STATE_ZOOM_SL: 2}
+#----------------------
+# TODO: Add to viewer_slice.py:
+
+    ps.Publisher().subscribe(self.OnSetMode, 'Set slice mode')
+
+def OnSetMode(self, pubsub_evt):
+    mode = pubsub_evt.data
+    # according to mode, set cursor, interaction, etc
+#----------------------
+# TODO: Add GUI classes (frame, tasks related to slice, toolbar):
+
+# always bind to this class (regarding slice mode) and not to
+# viewer_slice directly
+
+# example - pseudo code
+def OnToggleButtonSpin(self, evt)
+    if evt.toggle: # doesn't exist, just to illustrate
+        ps.Publisher().sendMessage('Enable mode', const.SLICE_STATE_ZOOM)
+    else:
+        ps.Publisher().subscribe('Disable mode', const.SLICE_STATE_ZOOM)
+
+
+#----------------------
+
+
+import constants as const
+
+class SliceMode(object):
+# don't need to be singleton, only needs to be instantiated inside
+# (Controller) self.slice_mode = SliceMode()
+
+    def __init__(self):
+        self.stack = {}
+
+        # push default value to stack
+        self.stack[const.LEVEL[const.SLICE_STATE_DEFAULT]] = \
+                    const.SLICE_STATE_DEFAULT
+
+        # bind pubsub evt
+        self.bind_events()
+
+    def _bind_events(self):
+        ps.Publisher().subscribe(self.OnEnableState, 'Enable mode')
+        ps.Publisher().subscribe(self.OnDisableState, 'Disable mode')
+
+    def OnEnableState(self, pubsub_evt):
+        state = pubsub_evt.data
+        self.AddState(state)
+
+    def OnDisableState(self, pubsub_evt):
+        state = pubsub_evt.data
+        self.RemoveState(state)
+
+    def AddState(self, state):
+        level = const.LEVEL[state]
+        max_level = max(self.stack.keys())
+
+        # Insert new state into stack
+        self.stack[level] = state 
+
+        # Only will affect InVesalius behaviour if it is the highest
+        # level in stack
+        if level == max_level:
+            # let viewer slice and other classes know this
+            # change (cursor, interaction, etc)
+            ps.Publisher().sendMessage('Set slice mode', state)
+
+    def RemoveState(self, state):
+        level = const.LEVEL[state]
+        max_level = max(self.stack.keys())
+
+        # Remove item from stack
+        self.stack.popitem(level)
+
+        # New max level
+        new_max_level =  max(self.stack.keys())
+
+        # Only will affect InVesalius behaviour if the highest
+        # level in stack has been removed
+        if level == max_level:
+            new_state = self.stack[new_max_level]
+            ps.Publisher().sendMessage('Set slice mode', state)
+             
diff --git a/invesalius/session.py b/invesalius/session.py
new file mode 100644
index 0000000..0be7c26
--- /dev/null
+++ b/invesalius/session.py
@@ -0,0 +1,33 @@
+# session.py
+
+
+class Session(object)
+    def __init__(self):
+        self.project_state = const.PROJ_FREE
+        # const.PROJ_FREE
+        # const.PROJ_OPEN
+        # const.PROJ_CHANGE
+        self.language = const.DEFAULT_LANG # en
+        self.imagedata_reformat = const.DEFAULT_REFORMAT # 1
+        self.layout = const.DEFAULT_LAYOUT #
+                                           # const.LAYOUT_RAPID_PROTOTYPING,
+                                           # const.LAYOUT_RADIOLOGY,
+                                           # const.LAYOUT_NEURO_NAVIGATOR,
+                                           # const.LAYOUT_ODONTOLOGY
+
+
+
+
+# USE PICKLE / PLIST !!!!!!
+# external - possibly inside controller
+#    def SetFileName(self, filename)
+#        # try to load config file from home/.invesalius
+#        load = self.Load(filename)
+#            if load:
+#                config = InVesaliusConfig
+        
+#    def Load(self, filename):
+#        # TODO: which file representation?
+#        # config?
+#        # plist?
+#        pass
--
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