diff --git a/index.yaml b/index.yaml
index c5358245..43b87e05 100644
--- a/index.yaml
+++ b/index.yaml
@@ -6,8 +6,8 @@ defs:
   AU: ['au.yaml']
   BE_FR: ['be_fr.yaml']
   BE_NL: ['be_nl.yaml']
-  BR: ['br.yaml']
   BG: ['bg.yaml']
+  BR: ['br.yaml']
   BY: ['by.yaml']
   CA: ['ca.yaml', 'northamericainformal.yaml']
   CH: ['ch.yaml']
@@ -17,43 +17,40 @@ defs:
   CR: ['cr.yaml']
   CY: ['cy.yaml']
   CZ: ['cz.yaml']
-  DK: ['dk.yaml']
   DE: ['de.yaml']
-  ECBTARGET: ['ecbtarget.yaml']
+  DK: ['dk.yaml']
   EE: ['ee.yaml']
   ES: ['es.yaml']
-  FederalReserve: ['federalreserve.yaml']
-  FederalReserveBanks: ['federalreservebanks.yaml']
-  FedEx: ['fedex.yaml']
   FI: ['fi.yaml']
   FR: ['fr.yaml']
   GB: ['gb.yaml']
   GE: ['ge.yaml']
   GR: ['gr.yaml']
-  HR: ['hr.yaml']
   HK: ['hk.yaml']
+  HR: ['hr.yaml']
   HU: ['hu.yaml']
   IE: ['ie.yaml']
   IL: ['il.yaml']
   IN: ['in.yaml']
   IS: ['is.yaml']
   IT: ['it.yaml']
+  JP: ['jp.yaml']
   KE: ['ke.yaml']
   KR: ['kr.yaml']
   KZ: ['kz.yaml']
   LI: ['li.yaml']
   LT: ['lt.yaml']
+  LU: ['lu.yaml']
   LV: ['lv.yaml']
   MA: ['ma.yaml']
   MC: ['mc.yaml']
-  MT_MT: ['mt_mt.yaml']
   MT_EN: ['mt_en.yaml']
+  MT_MT: ['mt_mt.yaml']
   MX: ['mx.yaml', 'northamericainformal.yaml']
-  NERC: ['nerc.yaml']
+  MY: ['my.yaml']
+  NG: ['ng.yaml']
   NL: ['nl.yaml']
-  LU: ['lu.yaml']
   "NO": ['no.yaml']
-  NYSE: ['nyse.yaml']
   NZ: ['nz.yaml']
   PE: ['pe.yaml']
   PH: ['ph.yaml']
@@ -64,24 +61,29 @@ defs:
   RS_LA: ['rs_la.yaml']
   RU: ['ru.yaml']
   SE: ['se.yaml']
+  SG: ['sg.yaml']
+  SI: ['si.yaml']
+  SK: ['sk.yaml']
+  TH: ['th.yaml']
   TN: ['tn.yaml']
   TR: ['tr.yaml']
-  TSX: ['tsx.yaml']
   UA: ['ua.yaml']
   US: ['us.yaml', 'northamericainformal.yaml']
-  UnitedNations: ['unitednations.yaml']
-  UPS: ['ups.yaml']
+  VE: ['ve.yaml']
+  VN: ['vn.yaml']
   ZA: ['za.yaml']
+  # Meta regions
+  Europe: ['at.yaml', 'be_fr.yaml', 'be_nl.yaml', 'ch.yaml', 'cz.yaml', 'dk.yaml', 'de.yaml', 'es.yaml', 'fr.yaml', 'gb.yaml', 'gr.yaml', 'hr.yaml', 'hu.yaml', 'ie.yaml', 'is.yaml', 'it.yaml', 'li.yaml', 'lt.yaml', 'lv.yaml', 'nl.yaml', 'no.yaml', 'pl.yaml', 'pt.yaml', 'ro.yaml', 'sk.yaml', 'si.yaml', 'bg.yaml', 'ua.yaml']
   NorthAmerica: ['ca.yaml', 'mx.yaml', 'us.yaml', 'northamericainformal.yaml']
-  SouthAmerica: ['ar.yaml', 'br.yaml', 'cl.yaml', 'co.yaml', 'pe.yaml', 've.yaml']
   Scandinavia: ['dk.yaml', 'is.yaml', 'no.yaml', 'se.yaml', 'fi.yaml']
-  Europe: ['at.yaml', 'be_fr.yaml', 'be_nl.yaml', 'ch.yaml', 'cz.yaml', 'dk.yaml', 'de.yaml', 'es.yaml', 'fr.yaml', 'gb.yaml', 'gr.yaml', 'hr.yaml', 'hu.yaml', 'ie.yaml', 'is.yaml', 'it.yaml', 'li.yaml', 'lt.yaml', 'lv.yaml', 'nl.yaml', 'no.yaml', 'pl.yaml', 'pt.yaml', 'ro.yaml', 'sk.yaml', 'si.yaml', 'bg.yaml', 'ua.yaml']
-  JP: ['jp.yaml']
-  VE: ['ve.yaml']
-  VN: ['vn.yaml']
-  SK: ['sk.yaml']
-  SI: ['si.yaml']
-  SG: ['sg.yaml']
-  MY: ['my.yaml']
-  TH: ['th.yaml']
-  NG: ['ng.yaml']
+  SouthAmerica: ['ar.yaml', 'br.yaml', 'cl.yaml', 'co.yaml', 'pe.yaml', 've.yaml']
+  # Organisations
+  ECBTARGET: ['ecbtarget.yaml']
+  FederalReserve: ['federalreserve.yaml']
+  FederalReserveBanks: ['federalreservebanks.yaml']
+  FedEx: ['fedex.yaml']
+  NERC: ['nerc.yaml']
+  NYSE: ['nyse.yaml']
+  TSX: ['tsx.yaml']
+  UnitedNations: ['unitednations.yaml']
+  UPS: ['ups.yaml']
diff --git a/lib/validation/index_validator.rb b/lib/validation/index_validator.rb
index fea86a62..40ef6264 100644
--- a/lib/validation/index_validator.rb
+++ b/lib/validation/index_validator.rb
@@ -19,6 +19,7 @@ class Index
       def call(index, definition_files)
         defs = validate_structure!(index)
 
+        validate_order!(defs.keys)
         validate_file_names!(defs)
         validate_all_files_indexed!(defs, definition_files)
         validate_all_indexed_files_exist!(defs, definition_files)
@@ -47,6 +48,31 @@ def validate_structure!(index)
         defs
       end
 
+      def validate_order!(order)
+        expected_order = order.sort_by do |name|
+          group = case name
+          when /\A[A-Z]{2}(\z|_)/
+            0 # countries
+          when *AGGREGATE_REGIONS
+            1 # meta regions
+          else
+            2 # organisations
+          end
+
+          [group, name.downcase]
+        end
+
+        unless order == expected_order
+          out_of_order = order.each_index.select{ |i| order[i] != expected_order[i] }
+
+          diff = out_of_order.chunk_while{ |a, b| a + 1 == b }.flat_map do |chunk|
+            chunk.map{ |i| "-#{order[i]}" } + chunk.map{ |i| "+#{expected_order[i]}" }
+          end
+
+          raise Errors::InvalidIndex.new("index has wrong order of definitions:\n#{diff.join("\n")}")
+        end
+      end
+
       def validate_file_names!(defs)
         invalid = indexed_files(defs).reject { |file| file =~ VALID_FILE_NAME }
 
diff --git a/spec/validation/index_validator_spec.rb b/spec/validation/index_validator_spec.rb
index e0a56555..ade2e8ea 100644
--- a/spec/validation/index_validator_spec.rb
+++ b/spec/validation/index_validator_spec.rb
@@ -4,7 +4,7 @@
 describe Definitions::Validation::Index do
   subject { described_class.new }
 
-  let(:index) { { 'defs' => { 'US' => ['us.yaml'], 'CA' => ['ca.yaml'] } } }
+  let(:index) { { 'defs' => { 'CA' => ['ca.yaml'], 'US' => ['us.yaml'] } } }
   let(:definition_files) { ['us.yaml', 'ca.yaml'] }
 
   context 'valid' do
@@ -25,14 +25,14 @@
     it 'allows aggregate regions to have no definition file of their own' do
       index = {
         'defs' => {
+          'BR' => ['br.yaml'],
+          'FR' => ['fr.yaml'],
+          'NO' => ['no.yaml'],
+          'US' => ['us.yaml'],
           'Europe' => ['fr.yaml'],
           'NorthAmerica' => ['us.yaml'],
           'Scandinavia' => ['no.yaml'],
           'SouthAmerica' => ['br.yaml'],
-          'FR' => ['fr.yaml'],
-          'US' => ['us.yaml'],
-          'NO' => ['no.yaml'],
-          'BR' => ['br.yaml'],
         }
       }
       expect(subject.call(index, ['fr.yaml', 'us.yaml', 'no.yaml', 'br.yaml'])).to be true
@@ -89,6 +89,66 @@
     end
   end
 
+  context 'invalid order' do
+    it 'raises error if the country codes are not ordered' do
+      index = { 'defs' => { 'US' => ['us.yaml'], 'CA' => ['ca.yaml'] } }
+      expect { subject.call(index, ['us.yaml', 'us.yaml']) }.to raise_error(Definitions::Errors::InvalidIndex) { |e|
+        expect(e.message).to eq("index has wrong order of definitions:\n-US\n-CA\n+CA\n+US")
+      }
+    end
+
+    it 'raises error if meta regions are not ordered' do
+      index = {
+        'defs' => {
+          'FR' => ['fr.yaml'],
+          'US' => ['us.yaml'],
+          'NorthAmerica' => ['us.yaml'],
+          'Europe' => ['fr.yaml'],
+        }
+      }
+      expect { subject.call(index, ['fr.yaml', 'us.yaml']) }.to raise_error(Definitions::Errors::InvalidIndex) { |e|
+        expect(e.message).to eq("index has wrong order of definitions:\n-NorthAmerica\n-Europe\n+Europe\n+NorthAmerica")
+      }
+    end
+
+    it 'raises error if organisation names are not ordered' do
+      index = {
+        'defs' => {
+          'Foo' => ['foo.yaml'],
+          'Bar' => ['bar.yaml'],
+        }
+      }
+      expect { subject.call(index, ['foo.yaml', 'bar.yaml']) }.to raise_error(Definitions::Errors::InvalidIndex) { |e|
+        expect(e.message).to eq("index has wrong order of definitions:\n-Foo\n-Bar\n+Bar\n+Foo")
+      }
+    end
+
+    it 'raises error if meta regions precede country codes' do
+      index = {
+        'defs' => {
+          'Europe' => ['fr.yaml'],
+          'FR' => ['fr.yaml'],
+        }
+      }
+      expect { subject.call(index, ['fr.yaml']) }.to raise_error(Definitions::Errors::InvalidIndex) { |e|
+        expect(e.message).to eq("index has wrong order of definitions:\n-Europe\n-FR\n+FR\n+Europe")
+      }
+    end
+
+    it 'raises error if organisation precede meta regions' do
+      index = {
+        'defs' => {
+          'FR' => ['fr.yaml'],
+          'Foo' => ['foo.yaml'],
+          'Europe' => ['fr.yaml'],
+        }
+      }
+      expect { subject.call(index, ['fr.yaml', 'foo.yaml']) }.to raise_error(Definitions::Errors::InvalidIndex) { |e|
+        expect(e.message).to eq("index has wrong order of definitions:\n-Foo\n-Europe\n+Europe\n+Foo")
+      }
+    end
+  end
+
   context 'invalid file names' do
     it 'raises error if an indexed file is uppercase' do
       index = { 'defs' => { 'US' => ['US.yaml'] } }
@@ -130,14 +190,14 @@
 
   context 'indexed files that do not exist' do
     it 'raises error if the index lists a file that is not on disk' do
-      index = { 'defs' => { 'US' => ['us.yaml'], 'HK' => ['hk.yaml'] } }
+      index = { 'defs' => { 'HK' => ['hk.yaml'], 'US' => ['us.yaml'] } }
       expect { subject.call(index, ['us.yaml']) }.to raise_error(Definitions::Errors::InvalidIndex) { |e|
         expect(e.message).to eq("index.yaml lists files that do not exist: hk.yaml")
       }
     end
 
     it 'reports every missing file, sorted' do
-      index = { 'defs' => { 'US' => ['us.yaml', 'zz.yaml'], 'HK' => ['hk.yaml'] } }
+      index = { 'defs' => { 'HK' => ['hk.yaml'], 'US' => ['us.yaml', 'zz.yaml'] } }
       expect { subject.call(index, ['us.yaml']) }.to raise_error(Definitions::Errors::InvalidIndex) { |e|
         expect(e.message).to eq("index.yaml lists files that do not exist: hk.yaml, zz.yaml")
       }
@@ -154,14 +214,14 @@
     end
 
     it 'reports every mismatched region, sorted' do
-      index = { 'defs' => { 'US' => ['ca.yaml'], 'GB' => ['ca.yaml'] } }
+      index = { 'defs' => { 'GB' => ['ca.yaml'], 'US' => ['ca.yaml'] } }
       expect { subject.call(index, ['ca.yaml']) }.to raise_error(Definitions::Errors::InvalidIndex) { |e|
         expect(e.message).to include("index entries do not list their own definition file: GB (expected 'gb.yaml'), US (expected 'us.yaml')")
       }
     end
 
     it 'raises error for a region that is not a known aggregate' do
-      index = { 'defs' => { 'Asia' => ['jp.yaml'], 'JP' => ['jp.yaml'] } }
+      index = { 'defs' => { 'JP' => ['jp.yaml'], 'Asia' => ['jp.yaml'] } }
       expect { subject.call(index, ['jp.yaml']) }.to raise_error(Definitions::Errors::InvalidIndex) { |e|
         expect(e.message).to include("Asia (expected 'asia.yaml')")
       }