Tips on 10.2 Jaguar, 10.3 Panther, 10.4 Tiger, 10.5 Leopard (End of the line for PowerPC Macs), 10.6 Snow Leopard, 10.7 Lion, 10.8 Mountain Lion and 10.9 Mavericks
Note: this tip, and the series of tips I've written from 10.3 to 10.9 all refer to Mac OS X client. The server edition may have different limitations. Please ask on the appropriate Server forum if you are interested in Mac OS X Server.
When determining your requirements for the operating system, see this tip: https://discussions.apple.com/docs/DOC-6413 for your Mac model and age.
Be sure to backup your data first at least twice before installing any operating system. Shut down, and disconnect any peripherals before continuing with the installation. Read the info below to ensure you are compatible.
Not many will be updating directly from Mac OS X 10.1.5 or earlier to 10.5, but those who are should read this article on firmware updates.
Finally, you may need to use the Startup Manager to boot the operating system when the 'C' key doesn't work in order to get the installer to work or repair the disk before installation if the initial attempt to install fails. To determine if that repair is necessary, post to the forum, and someone will be able to help you to find out which repairs might be necessary.
Leopard (10.5) is not to be confused with Snow Leopard (10.6), which are totally different paid operating systems.
Flashback affected users in 10.5 can now rejoice. Apple has released a removal software, and patch for 10.5.8 on Intel Macs.
PowerPC users may generally not be a target, however, to be on the safe side, it still is recommended you disable Java. If you must install Java on 10.5 because you have a PowerPC Mac that only supports 10.5.8, the latest Java for 10.5.8 tip explains how to optimize your Java for 10.5.8. Otherwise if you have an Intel Mac, read how to upgrade to 10.6.8, which supports a better Java, although still not the latest Java.
Also don't install any software from a website not belonging to a vendor, unless recommended by people on the forum.
Ask first if uncertain.
Mac OS X 10.5 (Leopard) was release October 26, 2007. Some deciding to upgrade to Leopard, may benefit more from upgrading to Tiger first until all their applications have been upgraded. Use this tip to learn how. 10.5's 10.5.8 update is the newest operating system available for PowerPC Macs (G4 with 867 Mhz, 1Ghz or higher, and G5) that meet its requirements. Some G5 towers run better with this firmware update when installed before updating the version of Mac OS X that shipped with them. 10.5 is available occasionally by calling Apple online store's phone number, even though the website does not show it. Many want to upgrade to Leopard because of i-Devices (iPhone, iPad, iPod) bought saying iTunes 10 or later is recommended. To check if that really is required, look at the i-Device requirements section of this tip.
If your Mac is Intel, and running 10.4.11 or earlier, and meets the hardware requirements of 10.6 (Snow Leopard) , a less expensive upgrade solution is to upgrade to 10.6 directly. No need to upgrade to 10.5 unless your software won't run in Snow Leopard. And even some newer Intel Macs, can install 10.7 (from 10.6.6, or USB Flash drive version of 10.7 on an erase and install), and the newest can install 10.8 or 10.9 from 10.6.8. Use aforementioned 10.6 & 10.7 links to determine which might be better for you.
and came in 10.5(.0 on October 26, 2007), 10.5.1 (November 15, 2007), 10.5.4 (June 30, 2008), and 10.5.6 (December 15, 2008) releases. It does not say Upgrade, Dropin, or OEM. If your Mac is older than 10.5.6 you can at minimum use the 10.5.6 release. As long as it meets the requirements below, and is older than the date of those releases, it can use all of them. Macs newer than December 15, 2008 must use either the installer disc that shipped with them, or Snow Leopard installer. The disc that shipped with Macs with 10.5 is increasingly more difficult to find, and matching it up with a Mac, though this article can help identify which one it may be.
on Macintouch covers Leopard incompatible software.
If part of your installing Leopard is bringing over data from a PowerPC Mac to an Intel Mac follow this tip.
Lastly, Apple has a listing of compatible printers.
For Leopard you need
867 Mhz on a single processor or faster (1 Ghz and higher are faster).
256 MB of RAM (preferably 512MB of more based on arbitrary experience).
9 GB of hard disk space (arbitrary experience adds another 15%) free.
Macs newer than August 28, 2009 can't run Leopard, and require a minimum of Snow Leopard.
#Mac OS X 10.5 is the earliest version of Mac OS X that will run iTunes 10. This is necessary for numerous i-Devices (iPad, iPhone, and iPods). To find out which require it, see this article
Should you require only iTunes 9, and are running a minimum of 10.4, backup your data at least twice, check in Apple menu -> About This Mac if you have a G3, G4, G5 (which is PowerPC), or Intel Core (which is Intel) make sure you are running 10.4.11 PowerPC or 10.4.11 Intel (depending on the aforementioned processors), and download iTunes 9.2.1. If you came here from running 10.3.9 or earlier read this tip on upgrading to Tiger first. The only reason you should upgrade to 10.5 is if some other software is required.
Note upgrading from an older Mac with only USB 1.1, one also has to consider only a few iPods were reasonably fast with those older Macs, or could get
power from them while connected.
Dual booting Mac OS 9 and Mac OS X 10.5, check this tip:https://discussions.apple.com/docs/DOC-1757
Classic is not compatible with 10.5. 10.4.11 on a PowerPC Mac (G3, G4, or G5) or earlier is needed for Classic. Intel Macs do not support
Classic at all.
Boot Camp is only available on Intel Macs with 10.5 and higher. For more on this limitation, see Does any version of 10.5 offer Boot Camp for PowerPC Macs?
Time Machine backup software first appeared in 10.5
You have several options of installation which are explained here: https://discussions.apple.com/docs/DOC-2209
10.5 has these combo updates which can be installed after backing up your data, and dismounting and disconnecting any data peripherals:
which some software may require as a minimum to be compatible with. 10.5.8 though is the only one where the most current security updates are available.
Combo updates may be installed on the same operating system version which was updated with Software Update, or the Delta update, to ensure no components are missing, as well as any version of 10.5 that is older.
All of which I recommend you backup your data at least twice before upgrading, and have sufficient disk space to install, and all your third party applications and drivers are known to be Leopard compatible, and after backing up your data you dismount and disconnect all but Apple keyboard and mouse and if there is an external display, that too. And just before you install repair permissions.
If any of the above terms confuse you, ask on the Discussions board what they mean, and one of the people who knows will gladly assist you.