Classical homological algebra takes place in additive categories. In homotopy theory such additive categories arise as homotopy categories of "additive groupoid enriched categories", in which a secondary analog of homological algebra can be performed. We introduce secondary chain complexes and secondary resolutions leading to the concept of secondary derived functors. As a main result we show that the E 3 -term of the Adams spectral sequence can be expressed as a secondary derived functor. This result can be used to compute the E 3 -term explicitly by an algorithm.