How would I achieve this in theory? I'm thinking about games like Warcraft 3 or Diablo 2 where you move your character by clicking on the ground. There are two ways I can think of how this would work. In the case of Diablo 2 it's fairly simple because the game is 2d. Here you might be able to get away with just doing an intersect test against a plane at the character's feet. As for Warcraft 3 or World of Warcraft, the world is 3d and you need to allow the character to move to the exact location (polygon?) he clicks on. You might then move the character toward the x/z coordinates of that polygon, while the collision system takes care of the y coordinate. I don't know. I'm interested in how to do this in 3d (2d advice is welcome though). What I have come up with so far is to iterate through all of the triangles in the partition of space where the player clicks, then do an intersection test to find out which triangle was clicked on and furthermore to get the exact location on the triangle (where the unprojected mouse ray intersects) to move the character to. I made a simple project to demonstrate the intersection method. It might be useful to someone beyond my intended purpose. So if world geometry was divided into small groups of triangles, and supposing my project demonstrates one of these groups, would this be scalable enough to use in a game engine?

use A* path finding to find the closest unobstructed node, and just make the character walk there first following the path, then when close enough directly to the point. The walking is entirely dependent on your character physics. You can have a look at this classic example for inspiration.

Steering Behaviors For Autonomous Characters

Raypicking against groups of triangles or bounding boxes seems like the way to go for determining the destination. I will have to consider how pathfinding fits into a scene management system and develop it alongside those systems so it works nicely.

Thanks for the great replies.