Time travel: the ultimate Wish Fulfillment; the desire to go back in time with the winning lottery number, or to meet your long lost Dream Girl, or simply to catch that barstid who stole your bicycle. Of all the genres and sub-genres in Spec Fi, time travel is probably the most popular.

It can also be one of the most challenging to get right. One problem with genre fiction writing is to avoid unexpected plot twists and loose ends which can make a mess of your story line. Time travel revels in these glitches, known as 'paradoxes', and relies on them heavily to forward the story. This makes time travel into something of a Mystery genre in which the protagonists must sort out the various realities and put them in some sort of order. (The butler did it!)

This can be a real challenge depending on how sensitive your reality is to time travelers mucking about. If every time trip creates a paradox, your genre reality will soon become unmanagable. On the other hand, since paradoxes are the genre's stock in trade, you need to define how these come to be, what the consequences are, and how your protagonists react to them. This can be a neat balancing act, since the actions of characters in each paradox can create still more paradoxes. (Paradoxii?)

As for reading these stories, all these paradoxes can lead to confusion. This is not a bad thing since an apparently tangled reality is part of the game, but you must sort it all out by the last page. Your paradoxes need to be closed out, explained away, or written off by 'The End' or your readers will not be happy.

Broadly speaking there are two ways to approach a time travel story:

The Top Down Approach: in which your protagonists use their time machine to visit the past, creating paradoxes, upsetting the temporal apple cart, and being a general nuisance until the Time Cops have to break up the party.

The Bottom Up Approach: which localizes on the 'receiving end' of a time jump and how the good folks back then deal with this bizarre, unexpected event. This has the advantage of reducing the number of paradoxes and limiting (or even eliminating) the need for advanced tech. It also places the major emphasis on character interactions, particularly conflicts between the locals and the travelers, which is essential to good story telling.

An excellent example of good (Bottom Up) time travel has to be 'The Guns Of The South', by Harry Turtledove (in which General Robert E. Lee receives a wonderful and unexpected gift: 100,000 AK 47s. Ah, but beware the hidden reasoning behind this 'gift', Marse Bob!) I recommend you read it before pursuing 'time travel' any further.

"Paradoxes can be paradoctored." (Robert A. Heinlein)