Permission to fish

Always make sure you have the correct permissions before going fishing, otherwise you could be committing an offence. For salmon & sea trout, it is a criminal offence to fish without written permission from the owner of the fishing rights, or his agent. For trout, it is a criminal offence to fish without permission where there is a Protection order in place, or where a loch is in single ownership. In all other cases, it is still a civil offence (the owner can prosecute you). For other types of freshwater fishing it is a criminal offence to fish without permission on a single ownership loch and a civil offence in all other cases.
For more details see the:
Salmon and Freshwater Fisheries (Consolidation) (Scotland) Act 2003.

Sea fishing, whether by boat or from the shore, no permission or permit is required.

Licences

Unlike England, you do not need a rod licence from the Government. There is one exception to this the river Border Esk which flows into England and is considered to be an English river for legal purposes, here you do need rod licence. The opposite is true on the Tweed, where the English part is considered to be Scottish for legal purposes, so no rod licence required.

Closed seasons

Salmon and trout fishing are subject to a close season during which it is illegal to fish. Close seasons vary from river to river. The earliest Salmon River to open is the Helmsdale on January 11th and the latest to close is the Tweed on 30th November. Most salmon rivers open sometime in February and close sometime in October. River trout fishing dates are 15th March to 6th October.
There is no legal close season for rainbow trout, grayling, coarse or sea fishing, although some fisheries do not operate in the winter.

Sundays

For salmon or sea trout fishing in Scotland, Sunday is a closed time and it is illegal to fish then. Though you are not stuck for fishing, as most commercial fisheries operate on Sundays.

Tackle

The requirements vary according to species being fished for and the area of Scotland in which you are fishing. Fishing is legally restricted to rod and line only, but many rivers have extra legal restrictions, such as the Tweed where you can only fish by fly for much of the season. Other rivers have conservation codes which mean that fishery owners will ban the use of spinners, prawns, worms etc for all or part of the season.