View Filesystem Access in Real Time on Mac OS X

You can use lsof or fs_usage in Terminal. lsof can be quite powerful. If it's about a specific process, you can try sampling it in Activity Monitor.

For a GUI tool, my recommendation is fseventer:

More GUI-oriented utilities, which I haven't used:

FileMon

FS Spy

Less visual

https://diigo.com/0xcp0 for highlights from Brendan's blog » Top 10 DTrace scripts for Mac OS X (2011-10-10).

More visual

For more modern operating systems that are incompatible with fseventer, there's Apple Xcode Instruments –

– powerful, but (compared to fseventer) in some cases, Instruments may be too heavyweight.

For viewing what files are accessed in real time you can leverage fs_usage part of the FSEvents feature in Mac OS X or even dtrace itself.

Usage: sudo fs_usage [PID].

For more info, check out the man page