| Legion | Origin of Name | Date of Creation |
| Legio I Germanica | Province where it gained distinction under Tiberius | pre-Augustan(?) |
| I Adiutrix pia fidelis | Assistant, ie. raised to supplement the legionary strength | Nero |
| I Italica | Raised in Italy | Neri |
| I Macriana | Raised by Clodius Macer | Nero |
| I Flavia Minerva | After Minerva | Domitian |
| I Parthica | Raised for campaigns in the East | Severus |
| II Adiutrix pia fidelis | Assistant, ie. raised to supplement the legionary strength | Vespasian |
| II Augusta | Raised by Augustus | Augustan |
| II Italica pia | Raised in Italy | M. Aurelius in 165 |
| II Parthica | Raised for campaigns in the East | Severus |
| II Traiana fortis | Strong, raised by Trajan | Trajan |
| III Augusta pia fidelis | Formed by Augustus | Augustan |
| III Cyrenaica | Province where it gained distinction | Pre-Augustan(?) |
| III Gallica | From the veterans of Caesar's Gallic legions | Pre-Augustan |
| III Italica concors | United, raised in Italy | M. Aurelius in 165 |
| III Parthica | Raised for campaigns in the East | Severus |
| IV Flavia firma | Steadfast, raised by Vespasian | Vespasian in AD 70 |
| IV Macedonica | Province where it gained distinction | Augustan |
| IV Scythica | Region where it gained distinction | Augustan |
| V Alaudae | The Lark | Pre-Augustan |
| V Macedonica | Province where it gained distinction | Pre-Augustan |
| VI Ferrata fidelis constans | 'Iron-sides', a nickname indicating endurance | Pre-Augustan |
| VI Victrix | Victorious, given after an outstanding victory | Pre-Augustan |
| VII Macedonica Claudia pia fidelis | For its loyalty to Claudius in 42 AD | Pre-Augustan |
| VII Gemina | One legion made out of two | Galba |
| VIII Augusta | Reconstituted by Augustus | Pre-Augustan |
| IX Hispana | Province where it gained distinction | Pre-Augustan |
| X Fretensis | From the naval war between Octavin and Sextus Pompeius | Pre-Augustan |
| X Gemina | One legion made out of two | Pre-Augustan |
| XI Claudia pia fidelis | For its loyalty to Claudius in 42 AD | Pre-Augustan |
| XII Fulminata | 'Lightning Hurler', probably gaiend under Caesar | Pre-Augustan |
| XIII Gemina pia fidelis | One legion made out of two | Augustan |
| XIV Gemina Martia Victrix | One legion made out of two | Augustan |
| XV Apollinaris | After the god Apollo | Augustan |
| XV Primigenia | After Fortuna Primigenia | Caligula or Claudius |
| XVI Flavia firma | Raised by Vespasian | Vespasian in 70 AD |
| XVI Gallica | Province where it gained distinction | Augustan |
| XX Valeria Victrix | Gained distinction under Valerius Messalinus | Augustan |
| XXI Rapax | Sweeping everything before it | Augustan |
| XXII Deiotariana | Raised by Deiotarus | Augustan |
| XXII Primigenia pia fidelis | After Fortuna Primigenia | Caligula or Cladius |
| XXX Ulpia victrix | Raised by Trajan and called victorious after serving with distinction in Dacia | Trajan |
| Contubernium | Tent party | 8 |
| Ten Contubernium = one Century | Basic unit of Imperial Legion | 80 |
| Two Centuries = one Maniple | Administrative/Training | 160 |
| Six Centuries = one Cohort | Main Battlefield/Tactical Unit | *480 |
| Ten Cohorts = a Legion | - | *5,120 |
*The First Cohort consisting of five double centuries 800 men
rather than the 480 in each of the remaining Cohorts.