I adore time travel stories. As far back as H.G. Wells and Mark Twain, the concept of time travel has given us the opportunity to examine how things change and how they stay the same. It is the ultimate fish-out-of-water scenario, and it's my favorite to write because the possibilities are limitless. Nearly any type of story can involve time travel. Take, for instance, "The Time Traveler's Wife" by Audrey Niffenegger, one of the most romantic stories I've ever read. Henry DeTamble, somewhat like Kurt Vonnegut's Billy Pilgrim in "Slaughterhouse Five," becomes unstuck in time and spends his life shifting back and forth between present, past and future. Sometimes knowing what will happen, and yet never knowing when, Henry examines his life from a rare perspective. But "The Time Traveler's Wife" is not categorized as science fiction, or even fantasy. Nor even romance. And that, to me, is a good thing. All fiction should be fluid of genre. In writing stories of time travel, the field of genre can be quite open, but I believe there are certain rules that must be followed, for the same reasons we adhere to spelling and grammar conventions. It aids communication. Not so much to be rigid about tropes, but for the story to make logical sense. As in any world building, regardless of genre, consistency is key. Of course I have my preferences, and I'll say right here that Tim Powers' "The Anubis Gates" is my bible. Powers begins with a mystical time transport mechanism and lays it over a quasi-scientific approach, and makes us believe his premise, which is that there are time portals that can be used to the advantage of those who know about them. The story is deliciously convoluted, yet it is so perfectly consistent internally that the reader can trust the world that has been built. Suspension of disbelief is effortless. In Powers' work, history does not change. That is, the story established at the beginning of the book is not changed by the actions of the time-traveling characters. That is the choice I also made in my first series, in which my main character, Dylan Matheson, is swept back in time to fulfill a destiny that had happened long before he was born. Quite the opposite of what was intended by the Scottish faerie who had brought him there. That is not to say that changing history is an inherently bad idea. Sometimes the story depends on history changing, which points out the interdependence of cause and effect. The concept of Butterfly Effect, though a mediocre movie, is a wonderfully broad area of exploration. But there are warning signs on the road up ahead. Changing the story makes for a complexity of cause and effect that can be disastrous. If the past is changed, then can the time travel have happened in the first place? It's the classic paradox: if I travel back in time and kill my grandfather, how could I have been born to travel back in time to kill my grandfather? Often it's good to steer clear of that particular paradox unless you're trying to solve it. And best of luck with that. Only one of my time travel stories changes history. "Kindred Spirits" involves a woman who is transported to the American Civil War by a spell she recites from an old diary. Late in the plotting process I realized that in changing the past I had removed the mechanism by which Shelby traveled to it. To fix that problem, I very carefully reconstructed the plot so that the mechanism remained and there was no paradox. Even so, there were still some things that appeared to be paradoxes. It required careful reading to follow both timelines and know where they diverged. Some readers follow it, and some don't. I also like to avoid making a character his own ancestor. Besides introducing a cause and effect paradox, I think it's a little creepy no matter how far back in the family the character is introducing himself. The best handing of this I've ever seen was in the movie "The Terminator." In one of the tightest screenplays I've ever seen, time traveler Kyle Reese arrives in present day to save the mother of his mentor, John Connor, and in the process becomes John's father. Maybe not a surprise ("Tell me about my son." "Well, he's about my height…"), but very, very cool. The last and not least important thing in writing time travel is to decide on the mechanism and stick with it. Decide where the portals are in both time and space, why they work, and how much control the characters have over them. If the portals travel in time, make them consistent and explain why they travel. If a character enters a portal at X time and travels back twenty years to Y time, spends three days and then travels back to X time plus three days, explain why the X portal has moved in time and the relationship between X and Y. Or, even worse, if the character travels again to the Y portal, if the Y portal moves it should be consistent with any movement of the X portal, and should be explained. And adding portals late in a series should be well supported and not done willy-nilly in either time or space. Indulging in hand-waving and making vague mutterings about "wormholes" doesn't cut it. This is basic world-building, and changing the rules late in the game just isn't fair to the reader. Be clear and consistent.