Unlike the anime, which seems to be doing its best to keep events in chronological order, the manga jumps all over the place time-wise so that the reader follows certain story arcs until they reach their conclusion or an appropriate break. All of which makes reading the manga after the anime a somewhat treacherous exercise...that's if you can actually stop reading in the first place.

But anyway, here's a rough comparison of events covered in both mediums (no spoilers)...

Note: the story is set in the '90s. According to the dates, it's apparently 1994.

E1 - ch00 (prologue)

E2 - ch1 (Yuuki Natsuno 01)

E3 - ch2 (Yuuki Natsuno 02)

-------volume 1 of manga

E4 - ch3 (Yuuki Natsuno 03)

E5 - ch4 (Yuuki Natsuno 04), ch5 (Masao extra 1)

-------volume 2 of manga. From here on, it gets a little confusing.

E6 - ch6 (Ozaki Toshio 01), most of ch7 (Yuuki Natsuno 05)

E7 - rest of ch7, half of ch8 (Yuuki Natsuno 06), ch12 (Masao extra 2)

E8 - rest of ch8, ch11 (Yuuki Natsuno 07)

E9 - ch9 (Ozaki Toshio 02), ch10 (Ozaki Toshio 03)

E10 - part of ch12, ch13 (Yuuki Natsuno 08)

--------volume 3/4 of manga

E11 - ch14-15 (Ozaki Toshio 04-05)

E12 - ch16-17 (Yuuki Natsuno 09-10)

--------volume 5 of manga

E13 - ch18 (Yuuki Natsuno 11), parts of ch19 (Ozaki Toshio 6), ch21 (Yuuki Natsuno 12)


Notes: Having only started looking at this recently (after episode 12, to be precise), I've had to rely on summaries, so it may need editing over the next few weeks. As I mentioned in a previous post, they may be aiming for a simultaneous anime/manga finish - but looking at 14's preview, I really think they might have wanted an extra episode (i.e. 23 instead of 22). Given how complicated it all is, I quite pity Sugihara Kenji, who's responsible for series composition...

---speculation for next episode...

E14 - ch20 (Ozaki Toshio: A certain doctor's record), rest of ch19, ch21(?)

--------volume 6 of manga

edit (2010-11-03): watched the preview for 14 a bit more carefully. We are definitely getting "A certain doctor's record" in the next episode...
final edit (2010-11-14): numbers changed to match the English releases