| Name | Character | Episodes |
|---|---|---|
| Jay O. Sanders | Narrator (voice) | 39 |
| Craig Sechler | Narrator | 19 |
| Neil Ross | Narrator | 17 |
| Liev Schreiber | Narrator (voice) | 16 |
| Will Lyman | Narrator | 12 |
| Stacy Keach | Narrator | 8 |
| David Ogden Stiers | Narrator | 8 |
| Brian Greene | Host | 7 |
| John Lithgow | Narrator | 6 |
| Peter Thomas | Narrator | 5 |
| Neil deGrasse Tyson | Self - Host | 5 |
| Neil Ross | 5 | |
| Richard Donat | Narrator | 5 |
| Liev Schreiber | 4 | |
| Corey Johnson | Narrator | 4 |
| David Attenborough | Narrator | 3 |
| Roy Scheider | Narrator | 3 |
| Demetri Goritsas | Narrator | 3 |
| Ted Marcoux | Narrator | 3 |
| Eric Meyers | Narrator | 3 |
| Stacy Keach | Narrator (voice) | 2 |
| Joe Morton | Narrator | 2 |
| Oliver Platt | Narrator | 2 |
| Valerie Harper | Self | 2 |
| Kathryn Walker | 2 | |
| Kathryn Walker | Narrator | 2 |
| Harrison Ford | Narrator | 1 |
| Sigourney Weaver | Narrator | 1 |
| John Hurt | Narrator | 1 |
| Keith David | Narrator | 1 |
| Hector Elizondo | Narrator | 1 |
| Rebecca De Mornay | Narrator | 1 |
| F. Murray Abraham | Narrator | 1 |
| Susan Sarandon | Narrator (voice) | 1 |
| Stacy Keach | 1 | |
| Shirley Henderson | Mileva Maric | 1 |
| Ian Holm | Narrator | 1 |
| Ian Holm | Narrator (voice) | 1 |
| Stockard Channing | Narrator (voice) | 1 |
| Stockard Channing | Self - Bible Reader | 1 |
| Derek Jacobi | Voice of Ernest Shackleton | 1 |
| Carl Sagan | 1 | |
| Scott Handy | Isaac Newton | 1 |
| Joe Morton | Additional Narration | 1 |
| Sally Kellerman | Narrator | 1 |
| Henry Ian Cusick | Charles Darwin | 1 |
| Samuel West | Humphry Davy | 1 |
| Anton Lesser | Voltaire | 1 |
| Hal Linden | Narrator | 1 |
| Martin Jarvis | G.H. Hardy (voice) | 1 |
| Martin Jarvis | Narrator (voice) | 1 |
| Valerie Harper | Narrator (voice) | 1 |
| Ruben Santiago-Hudson | Percy Julian | 1 |
| Simon Callow | Galileo | 1 |
| Frances O'Connor | Emma Darwin | 1 |
| Nigel Bennett | Sir Charles Lyell | 1 |
| Tony Robinson | Younger Ernie | 1 |
| Anita Carey | Joey's Mother | 1 |
| Courtney B. Vance | Narrator | 1 |
| Jim Carter | Narrator | 1 |
| Zachary Quinto | Narrator | 1 |
| Neil deGrasse Tyson | Self - Narrator | 1 |
| Will Lyman | Narrator (voice) | 1 |
| Darri Ingolfsson | Godfred Hansen | 1 |
| Steven Robertson | Michael Faraday | 1 |
| Aidan McArdle | Albert Einstein | 1 |
| Alex MacQueen | Chater | 1 |
| Julian Rhind-Tutt | Antoine Lavoisier | 1 |
| Joseph Campanella | Narrator (voice) | 1 |
| William Hootkins | Voice of Methuselah | 1 |
| William Franklyn | Narrator (voice) | 1 |
| Maureen Bennett | Lady Franklin | 1 |
| David Swift | Nikolai Vavilov | 1 |
| Emily Woof | Lise Meitner | 1 |
| John Benjamin Hickey | Narrator | 1 |
| John Fraser | Inquisitor | 1 |
| Maureen Stapleton | Narrator (voice) | 1 |
| Mark Wing-Davey | Joey's Father | 1 |
| Bohdan Poraj | Francis Crozier | 1 |
| David Suzuki | Narrator (voice) | 1 |
| Ty Glaser | Marie Anne Lavoisier | 1 |
| Bray Poor | Narrator | 1 |
| Salvatore Vecchio | Narrator | 1 |
| Forbes Collins | Iakovlev | 1 |
| Stephen Noonan | Marat | 1 |
| Noel Davis | Scientist | 1 |
| Jeremy Akerman | Parslow | 1 |
| Jan Maxwell | Narrator | 1 |
| Craig Sechler | Self - Narrator | 1 |
| Jere Shea | George Soper | 1 |
| Ray Magliozzi | Host | 1 |
| Cornelius Garrett | Voices of the Ambassador and Castelli | 1 |
| Alan Yentob | Narrator | 1 |
| William H. Whyte | Self | 1 |
| Thom Fell | James Fitzjames | 1 |
| Philip Herbert | Count de Amerval | 1 |