diff --git a/array2xls/gui.py b/array2xls/gui.py
index 7b19db1..c2e0bb5 100644
--- a/array2xls/gui.py
+++ b/array2xls/gui.py
@@ -4,6 +4,12 @@ from tkinter import filedialog
 
 import validators
 
+APP_STATE_1 = 'no valid files selected'
+APP_STATE_2 = 'no valid fields selected'
+APP_STATE_3 = 'ok, single files'
+APP_STATE_4 = 'ok, multiple files'
+
+
 class StatusPanel(tk.Frame):
 
     def __init__(self, parent):
@@ -48,12 +54,15 @@ class FieldPanel(tk.Frame):
         self._parent = parent
         self.is_single = tk.BooleanVar()
         self.is_single.set(True)
-        self.rbtn_single = tk.Radiobutton(self, text='single excel files', variable=self.is_single, value=True, command=self.disable_listbox)
-        self.rbtn_combine = tk.Radiobutton(self, text='one combined excel file', variable=self.is_single, value=False, command=self.enable_listbox)
+        self.rbtn_single = tk.Radiobutton(
+            self, text='single excel files', variable=self.is_single, value=True, command=self.disable_listbox)
+        self.rbtn_combine = tk.Radiobutton(
+            self, text='one combined excel file', variable=self.is_single, value=False, command=self.enable_listbox)
         self.rbtn_single.pack(anchor=tk.W)
         self.rbtn_combine.pack(anchor=tk.W)
-        self.listbox = tk.Listbox(self, selectmode=tk.MULTIPLE, activestyle='none')
-        self.listbox.pack(fill=tk.X, ipady=5, ipadx=5)
+        self.listbox = tk.Listbox(self, selectmode=tk.MULTIPLE, activestyle='none', height=19)
+        self.listbox.pack(fill=tk.X, ipady=5, ipadx=5, )
+        self.listbox.bind('<<ListboxSelect>>', self._parent.toggle_run_button)
         self.pack(side=tk.TOP, fill=tk.X)
 
     def disable_radio_btn(self):
@@ -62,6 +71,7 @@ class FieldPanel(tk.Frame):
 
     def disable_listbox(self):
         self.listbox.config(state="disabled")
+        self._parent.toggle_run_button()
 
     def disable(self):
         self.disable_listbox()
@@ -74,6 +84,7 @@ class FieldPanel(tk.Frame):
 
     def enable_listbox(self):
         self.listbox.config(state="normal")
+        self._parent.toggle_run_button()
 
     def enable(self):
         self.enable_listbox()
@@ -91,13 +102,12 @@ class FieldPanel(tk.Frame):
         self.disable_listbox()
 
 
-
 class ActionPanel(tk.Frame):
 
     def __init__(self, parent):
         tk.Frame.__init__(self, parent.master)
         self._parent = parent
-        self.btn_go = tk.Button(self, text="GO!", command=self._parent.quit)
+        self.btn_go = tk.Button(self, text="GO!", command=self._parent.convert_files)
         self.btn_go.pack(side=tk.LEFT, pady=5, padx=5)
         self.btn_quit = tk.Button(self, text="Quit", command=self._parent.quit)
         self.btn_quit.pack(side=tk.RIGHT, pady=5, padx=5)
@@ -113,10 +123,12 @@ class ActionPanel(tk.Frame):
 class Application(tk.Frame):
 
     def __init__(self, master):
-        master.minsize(height=330, width=200)
+        master.minsize(height=330, width=300)
         tk.Frame.__init__(self, master)
         self._master = master
         self.pack(fill=tk.BOTH)
+        self.file_validator = None
+        self.validated_files = None
         self.status_panel = StatusPanel(self)
         self.file_panel = FilePanel(self)
         self.field_panel = FieldPanel(self)
@@ -124,6 +136,7 @@ class Application(tk.Frame):
         self.reset()
 
     def reset(self):
+        self.status_panel.clear_text()
         self.field_panel.disable()
         self.action_panel.disable()
         self.file_validator = None
@@ -145,13 +158,53 @@ class Application(tk.Frame):
         self.validated_files = [f for f in validators.validate_files(file_selection, self.file_validator)]
         status = '%d of %d valid %s files found' % (len(self.validated_files), len(file_selection), self.file_validator.type)
         self.file_panel.set_text(status)
-        if len(self.validated_files) == 0:
-            self.reset()
-            return
+        if self._state == APP_STATE_1:
+            return self.reset()
         self.field_panel.enable_radio_btn()
         self.field_panel.set_listbox_content(self.file_validator.data_fields, self.file_validator.defaults)
+        self.toggle_run_button()
 
-
+    def toggle_run_button(self, event=None):
+        self.action_panel.disable()
+        if  self._state in (APP_STATE_3, APP_STATE_4):
+                self.action_panel.enable()
 
     def quit(self):
         self._master.destroy()
+
+    def convert_files(self):
+        state = self._state
+        if state not in (APP_STATE_3, APP_STATE_4):
+            return self.reset()
+        i = len(self.validated_files)
+        if state == APP_STATE_3:
+            text = 'Converting %d data files to %d excel files ...' % (i, i)
+            self.status_panel.set_text(text)
+            self._convert_to_single_files()
+        else:
+            text = 'Combining %d data files into one excel file ...' % i
+            self.status_panel.set_text(text)
+            self._combine_data_files()
+        self.status_panel.set_text(text + ' Done')
+
+    @property
+    def _state(self):
+        c = self.field_panel.rbtn_single.config()
+        if not self.validated_files or c['state'] == 'disabled':
+            return APP_STATE_1
+        elif self.field_panel.is_single.get():
+            return APP_STATE_3
+        elif self.field_panel.listbox.curselection():
+            return APP_STATE_4
+        else:
+            return APP_STATE_2
+
+    def _convert_to_single_files(self):
+        pass
+
+    def _combine_data_files(self):
+        pass
+
+
+
+