National Whale and Dolphin Watch weekend runs from Friday to Sunday.
Land watches will be taking place in Cornwall and Devon, and accredited boat operators who will be hoping to make off shore sightings
The species that monitors are hoping to see in the seas around the South West coast include bottlenose dolphin, harbour porpoise, short beaked common dolphin, Risso's dolphin, minke whale, and fin whale.
Land based watches are being held at Hella Point and Godrevy Point in Cornwall; Ilfracombe in North Devon; Berry Head Nature Reserve, Froward Head and Sharkham Point in South Devon.
Boat operators taking part are based in Penzance, Rock, Paignton and Dartmouth.
Organisers say that National Whale and Dolphin Watch is important because the health of our marine mammals is vital to maintaining the balance of marine ecosystems, providing an early indication of pollution or climate change issues.
In many areas of Scotland, Wales and in the South West of England responsible eco-tourism is also important to the local economy.
The information that is gathered feeds into a growing bank of information about the 29 species of cetaceans – whales, dolphins and porpoises – which have been known to visit the shores of Britain and Ireland.
The knowledge builds a better understanding of species' movements, their distribution and the health of populations, helping deliver effective conservation policies.
Many people in the UK are still unaware that dolphins and whales are off the UK coast all year round, and particularly from Spring to October. The watch therefore also helps raise public awareness of the diversity and fragility of the UK's marine mammal populations, say organisers.