Platform-Specific information about capture privileges
You need to run Wireshark or TShark on an account with sufficient privileges to capture, or need to give the account on which you're running Wireshark or TShark sufficient privileges to capture. The way this is done differs from operating system to operating system.
To be secure (at least in a way), it is recommended that even an administrator should always run in an account with (limited) user privileges, and only start processes that really need the administrator privileges. The ["Security"] page provides explanations why this is a good idea.
The WinPcap driver (called NPF) is loaded by Wireshark when it starts to capture live data.
This loading requires administrator privileges. Once the driver is loaded, every local user can capture from it until it's stopped again.
Note: Simply stopping Wireshark won't stop the WinPcap driver!
It might not be desirable that any local user can also capture from the network while the driver is loaded, but this can't be currently circumvented. Please note that this is not a limitation of the Wireshark implementation, but of the underlying WinPcap driver; see [http://www.winpcap.org/misc/faq.htm#Q-7 this note in the WinPcap FAQ].
There are three possible solutions to start Wireshark with the privilege to capture:
Start Wireshark as Administrator
Advantage: Very easy to work with.
Disadvantage: It's very unsecure running Wireshark this way as every possible Wireshark exploit will be running with the administrator account being able to compromise the whole system.
Start the NPF driver automatically at system start
The Wireshark installer has an option (default: off) to install the WinPcap driver automatically at system boot. You can change the start settings of the NPF service to "automatic" or "system". There are several ways to enable this.
From the Device Manager you can select View->Show hidden devices, then open Non-Plug and Play Drivers and right click on NetGroup Packet Filter Driver. In the driver properties you can set the startup type as well as start and stop the driver manually.
From the command line you can run
sc config npf start= auto(This must be run as Administrator under Vista.)
In the registry you can change HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\NPF\Start from 0x3 (SERVICE_DEMAND_START) to 0x2 (SERVICE_AUTO_START) or 0x1 (SERVICE_SYSTEM_START).
As the driver is already started you can run Wireshark as user all the time.
Advantage: Very easy to work with.
Disadvantage: Every local user can always capture live data.
Start the NPF driver by hand
You can start the driver by hand before starting Wireshark and stop it afterwards.
Using Wireshark running in a user account could look like:
Start the NPF driver:
runas /u:administrator "net start npf"
Start Wireshark as a user and work with it, including capturing, until the specific job is finished.
Stop the NPF driver again:
runas /u:administrator "net stop npf"
This can obviously be automated using a batch file.
Advantage: Most secure solution.
Disadvantage: You'll have to enter the password each time you start/stop Wireshark.
Running Wireshark (or any other network capture/analyzer, for that matter) on Linux needs root privileges. Therefore, you have to have root privileges when starting Wireshark, else you can't capture data. Please note that you don't have to login as root when starting your computer, you can use su(1) or sudo(8) for that purpose. However, this remains unsecure as the dissectors, the parts of Wireshark which parse the captured data, run with root privileges as they did before. A much safer solution would be to su(1) to root, then use the bundled dumpcap to dump the data (for example, you can evoke dumpcap by using "dumpcap -w ./dumpfile", which will dump the packets to the file "dumpfile" in the current working directory. See "dumpcap -h" for details). You could also use tcpdump for this purpose. The advantage of this solution is, while dumpcap/tcpdump still run as root, you can run Wireshark as a ordinary user and load the data you captured previously, so effectively this is kinda "privilege separation by hand".
BSD (including Mac OS X)
In order to capture packets, you must have read access to the BPF devices in /dev/bpf*.
On BSDs without a devfs, the special files for those devices are on your root file system, and changes to them will persist across reboots. In order to allow yourself, or yourself and others, to capture traffic without running Wireshark as root, either make them owned by you, or make them owned by a group to which you and others to whom you want to give capture permission belong and give that group read access, or, if your BSD supports ACLs on special files, add the users who should have permission to capture to the ACL, with the ACL entry giving them read permission. You will probably need super-user permission to do this.
On BSDs with a devfs (this includes Mac OS X), this might involve more than just having somebody with super-user access setting the ownership and/or permissions on the BPF devices - it might involve configuring devfs to set the ownership or permissions every time the system is booted, if the system supports that; FreeBSD 5.x's devfs does. If the system doesn't support that - Mac OS X's devfs doesn't, you might have to find some other way to make that happen at boot time, such as a command in one of the system rc files, or a startup item in OS X; see the ChmodBPF directory in libpcap 0.9.1 or later for such a startup item.
Any user can, in principle, capture network traffic. However, no user (not even the super-user) can capture in promiscuous mode on an interface unless the super-user has enabled promiscuous-mode peration on that interface using pfconfig(8), and no user (not even the super-user) can capture unicast traffic received by or sent by the machine on an interface unless the super-user has enabled copy-all-mode operation on that interface using pfconfig, so useful packet capture on an interface probably requires that either promiscuous-mode or copy-all-mode operation, or both modes of operation, be enabled on that interface. You might be able to limit the set of users allowed to capture traffic by changing the ownership and/or permissions of the /dev/pfilt* devices.