The general problem addressed in this work is the development of a systematic study of the thresholding greedy algorithm for general biorthogonal systems in quasi-Banach spaces from a functional-analytic point of view. If (xn, x * n ) ∞ n=1 is a biorthogonal system in X then for each x ∈ X we have a formal expansionxn. The thresholding greedy algorithm (with threshold ε > 0) applied to x is formally defined asxn. The properties of this operator give rise to the different classes of greedy-type bases. We revisit the concepts of greedy, quasi-greedy, and almost greedy bases in this comprehensive framework and provide the (non-trivial) extensions of the corresponding characterizations of those types of bases. As a by-product of our work, new properties arise, and the relations among them are carefully discussed.