Voltaire wrote "With great power comes responsibility", a quote which can easily be applied to scientists nowadays whose work effectively shapes the life of billions of living beings, operating through various disciplines from medicine through to ecology. To help scientists working with wild creatures, animal-attached electronic devices, commonly referred to as 'tags', have become indispensable tools, pushing the boundaries into the unimaginable enabling, for instance, information to be sent from animals into space and back via satellites. This 'great power' does indeed come with 'responsibility' however, as evidence piles up of the deleterious effects of tags on their animal carriers. The aim of this doctoral project is to provide scientists with an analytical framework within which to examine the effects of external tags on wild animals with a view to providing guidelines informing best practise in animal tagging. For that purpose, an integrative, multidisciplinary approach was undertaken which, from a theoretical to an experimental level, assessed the impact of tags on birds. With a main focus on marine birds, the results show that tag effects ranged from behavioural aberrations to compromised energetics, ultimately reducing both flying and swimming performance. This impact varied as a function of tag size, mass, shape, position and attachment, as well as being dependent on bird morphology and lifestyle. The length of time to which a bird is exposed to deleterious tag effects appears critical since these effects can snowball over time. Fortunately, and as reported in this thesis, there are simple rules which can be implemented to help minimise tag impact even for long-term studies, mainly through an optimised tag design and innovative attachment system. So, happily, this thesis shows that by careful thinking, we can benefit maximally from our 'great power' and thus ensure that our 'responsibilities' to wild animals are best informed.