For non-story-related encounters, I prepare stuff that is most likely to happen while the party is travelling or staying in a location, depending on time of day or even season of the the year. IOW, things that make sense for the time and place. I NEVER roll random encounters on the fly because it bogs down the game too much. I keep several generic "random" encounters on deck for that reason. For example, my current party is planning to travel overland from one city, through three small towns, to another small city. I've already determined (by die roll) that they're going to have an encounter along the road between the first and second small towns and my map shows that stretch of road passing near some mountains. It being the dead of winter with food getting a bit scarce, it's likely for some mountain beasties to come down to look for food. In this part of my world, that's going to be either wolves, dire wolves, or hill giants (no bandits because there's no merchant traffic around here in the middle of winter). Out of those three possibilities, I randomly picked hill giants, set up a map, tokens, random treasure, and planned the giant's strategy (taking into account that hill giants aren't very bright). If the party decides, mid-journey, to change plans and go somewhere else, the environment that they're travelling through is still close enough that this encounter will still likely work. Encounters that have something to do with the plot line that my players are currently following will have specific monsters or NPCs that are part of the story. Also, when I'm in the mood and have spare time, I create more challenging encounters and place them in set locations in my campaign. The players may or may not find these encounters, but they may run into them while travelling, or hear about them in tavern gossip or other information gathering sessions. I almost always save "exotic" monsters like undead, fey, etc. for set piece or plot related encounters. My players will never "just happen" to run into an Ogre Magi, Huli Jin, or young amethyst dragon. Vincent M. said: How do DMs generally generate their random encounters?