Maybe it's because I'm a huge fan of Terry Pratchett, but I have a hard time writing in chapters. I've been using Scrivener for the last couple of years for my writing projects, and it has made me think more about writing in scenes, rather than just word-vomiting across the page. While I've gotten better about determining when it's time to move from one scene to the next, I still find it difficult to figure out a chapter break.
I spent a lot of time studying theater in high school and college, so it comes a little easier for me to think in terms of "scenes." To me, a scene is defined as specific interaction or dialogue that moves to a defined purpose; ideally, it should also set up the purpose for the next scene, as well. For example, in the book I'm working on now (I've moved back to my South American alt-history, as I leave Paranormal Investigations to simmer for a bit), a scene begins when my main character walks into a room full of women to bring an artifact to a small girl. Once she has done that, the majority of the other women are dismissed, as there is going to be a private conversation between the main character and one of the women. That scene ends as the other women leave.
Chapters, on the other hand...there are definitely chapters that I've read that have felt like scenes - a single interaction, setting up the next one. However, more frequently I find chapters to be a combination of scenes that have something in common, such as location or the people interacting. The fact that chapter breaks are so fuzzy is what makes it harder for me, because it doesn't feel natural for me to "force" chapter breaks.
At this point, I'm still writing with scenes, as I've found that it helps me figure out the overall plot quite nicely. I'll probably spend some quality time figuring out chapter breaks during revision, but I'll most likely need some help in that regard.
How about you? Do you find it easier to write in chapters, in scenes, or in some other format completely? What led you to writing that way?