The first and second authors were partially supported by the Spanish Ministry of Economy and Competitiveness under the grants MTM2010-15831 and MTM2013- 42178-P, and by the Government of Catalonia under the grant SGR-119-2009. Thefirst author was also partially supported by the Spanish Ministry of Science and Innovation under a Ramón y Cajal research contract and by the Andalusian Ministry of Economy, Innovation and Science under the grant FQM-5713.

We extend Deligne's notion of determinant functor to Waldhausen categories and (strongly) triangulated categories. We construct explicit universal determinant functors in each case, whose target is an algebraic model for the 1-type of the corresponding K-theory spectrum. As applications, we answer open questions by Maltsiniotis and Neeman on the K-theory of (strongly) triangulated categories and a question of Grothendieck to Knudsen on determinant functors. We also prove additivity theorems for low-dimensional K-theory of (strongly) triangulated categories and obtain generators and (some) relations for various K1-groups. This is achieved via a unfied theory of determinant functors which can be applied in further contexts, such as derivators.