Read these tips for maximizing your download speed.

Some Internet Service Providers (ISPs) throttle peer-to-peer traffic, and even block it completely on well known peer to peer ports. If your ISP is listed on this page, it is likely you will experience these issues.

Transmission's encryption feature may overcome any ISP throttling. Checking the 'Ignore unencrypted peers' box (Prefs >> Advanced) also may improve your speed further, at the expense of losing some potential peers in the swarm. Changing the port Transmission uses might help if the ISP targets particular ports.

Ultimately, the speed you get depends on the quality of the peers you are downloading from. If they have dial up connections, you are only going to be able to download at dial up speeds. Furthermore, if there are few seeds and many peers, more people will be fighting for the same scarce pieces which will slow things down. Best results are achieved when the torrent has more seeds than peers.

Normally this is because the tracker is down, and thus Transmission is unable to interact with other peers. DHT (trackerless torrents) is not a feature at the moment, however is slated for a future release.

If there are no seeders in the swarm, and all the other peers have sent you what they have, you (and everyone else) will not be able to complete the download, and your speed will drop to zero.

Torrents take a while to get going and so may not download much (if at all) initially. Most torrents are downloading at some rate after 15 or so minutes.

Post the crash log on the support forums so that the issue can be fixed as quickly as possible. Crash logs are held in ~/Library/Logs/CrashReporter/

If your torrents' progress are incorrect when you reopen Transmission (eg they are starting from 0%) then you should manually recheck them. Click here for instructions.

PEX (Peer Exchange) is a method of discovering new peers via other peers, rather than the tracker. It allows Transmission to attach to the swarm much quicker. PEX is automatically disabled for privately tracked torrents. The feature is compatible with both Azureus and µTorrent peers.

Transmission encrypts the connections it makes with other peers when necessary, using the RC4 cipher. The implementation is compatible with other clients such as Azureus and µTorrent. It is always enabled, however you have two options: accepting connections from anyone, regardless of encryption; or only accepting encrypted connections. Note that the latter option may make you unconnectable in some swarms. The encryption feature does not mean your session is secure or anonymous, it is merely a way to avoid the traffic shaping measures some ISPs have implemented.

This is a constraint imposed on Transmission, as more than this number is unnecessary. A global limit is set at 200 peers.

Growl is a notification system. Lots of applications can interface with it.
http://growl.info/

Blue: we have this piece, lighter shades of blue indicate incomplete pieces
Green: connected peers have this piece (the darker the green, the more there are)
Red: no connected peers have this piece

Peers are people you are connected to. If they have the whole file and are only uploading, they are referred to as a 'seeder'. If they only have part of the torrent, and are downloading and uploading, they are referred to as a 'leecher'.

This shows your upload/download ratio. 1.00 means you have uploaded as much as you have downloaded, 2.00 means you have uploaded twice the amount you have downloaded, etc.

Downloaded is all the data you have ever downloaded.
Verified is how much checksum-verified data you currently have. Unverified data is the incomplete pieces of the torrent that are waiting to become complete so they can be checked.
Have is the sum of verified and unverified data.
Failed DL is data that failed the checksum test and needs to be downloaded again.

'Nightlies' are releases on the bleeding edge of development. They normally contain new features and bugfixes, but are not officially supported (although you are more than welcome to discuss them on the Transmission forums). You can try one out here.