The original World of Darkness games were not designed as a unified whole, as a result, while the games share a common system, crossing over elements tends to require a highly involved Storyteller. In addition, the various supernatural entities of the World Of Darkness are fairly ignorant of each other, meaning that in canonical campaigns there should be minimal, if any interaction between the different groups - generally as a rare antagonist or a cameo. Crossover is used generically to refer to both mechanical or plot situations where characters from two different games interact. Few White wolf publications provide any guidelines for crossovers between the games set in the World of Darkness, these are:
Ghost Towns for Werewolf: The Apocalypse and Wraith: The Oblivion
Under A Blood Red Moon for Werewolf and Vampire: the Masquerade
Crossover problems are less important in the current World of Darkness since a common system was designed from the start.