NewbieRunner

There have been some changes in FRA since hch's very helpful and comprehensive post. The main change as far as I know is the introduction of the EU security regulations in Concourse B. All arrivals in Concourse B from a non-Schengen country must now clear security before connecting to a flight to a non-Schengen country from Concourse B.

Since I have successfully avoided FRA recently and most of my connections are from non-Schengen to non-Schengen in any case, my knowledge of FRA is not as extensive as some posters in this thread but here are some basic rules that I'm aware of.

If you arrive from a non-Schengen country* and connecting to any destination you will have to clear security. (*Strictly speaking this should read "non-EU" but the layout of FRA does not always allow it, e.g. arrivals from the UK. Also arrivals from the US should no longer need security check before a connecting flight but I'm not sure if this has been implemented.)

If you arrive from a Schengen country you should not need to go through security before a connecting flight.

You have to go through passport control between Schengen and non-Schengen flights.

You don't have to go through passport control between Schengen and Schengen or non-Schengen and non-Schengen flights.

Further security may be necessary if you use the SkyLine train between A and B Concourses.

Access to lounges in concourses other than where your flight departs could vary depending on the flexibility of security personnel and the level of security in force. The Tower Lounge, for example, used to be exclusively for US departures though sometimes this rule seems to be relaxed.

I'd be happy to post a more comprehensive list if someone can confirm or correct any details.