A question about LVM

Hi everyone. Is it possible to put the LVM layer on top of an existing filesystem which currently holds data? When I partitioned my drive while ago and installed the Reiser FS, I didn't know much about LVM and recently I read some stuff about it and am very interested to know if it can be done at this stage or not. I couldn't find an explicit reference to this question in the articles that I found about LVM. Thanks, Ali.