Falsifiability is a standard of evaluation of scientific theories and hypotheses. A hypothesis is falsifiable if it belongs to a language or logical structure capable of describing an empirical observation that contradicts it. It was introduced by the philosopher of science Karl Popper in his book The Logic of Scientific Discovery (1934). Popper emphasized that the contradiction is to be found in the logical structure alone, without having to worry about methodological considerations external to this structure. He proposed falsifiability as the cornerstone solution to both the problem of induction and the problem of demarcation.

Popper also emphasized the related asymmetry created by the relation of a universal law with basic observation statements and contrasted falsifiability with the intuitively similar concept of verifiability that was then current in the philosophical discipline of logical positivism. He argued that the only way to verify a claim such as "All swans are white" would be if one could empirically observe all swans, which is not possible. On the other hand, the observation of a single black swan is enough to refute this claim.

This asymmetry can only be seen rigorously when methodological falsification issues are put aside. Otherwise, a stated observation of one or even more black swans constitute at best a problematic refutation of the claim. Accordingly, to be rigorous, falsifiability is a logical criterion within an empirical language that is accepted by convention and allows these methodological considerations to be avoided. Only then the asymmetry and falsifiability are rigorous. Popper argued that it should not be conflated with falsificationism, which is a methodological approach where scientists actively try to find evidence to disprove theories. Falsifiability is distinct from Lakatos' falsificationism. Its purpose is to make theory predictive, testable and useful in practice.

By contrast, the Duhem–Quine thesis says that definitive experimental falsifications are impossible and that no scientific hypothesis is by itself capable of making predictions, because an empirical test of the hypothesis requires background assumptions, which acceptations are methodological decisions in Lakatos' falsificationism.

Popper's response was that falsifiability is a logical criterion. Experimental research has the Duhem problem and other problems, such as the problem of induction, but, according to Popper, logical induction is a fallacy and statistical tests, which are possible only when a theory is falsifiable, are useful within a critical discussion.

Popper's distinction between logic and methodology has not allowed falsifiability to escape some criticisms aimed at methodology. For example, Popper's rejection of Marxism as unscientific because of its resistance to negative evidence is a methodological position, but the problems with this position are nevertheless presented as a limitation of falsifiability. Others, despite the unsuccessful proposals of Russell, the Vienna Circle, Lakatos, and others to establish a rigorous way of justifying scientific theories or research programs and thus demarcating them from non-science and pseudoscience, criticize falsifiability for not following a similar proposal and for supporting instead only a methodology based on critical discussion.

As a key notion in the separation of science from non-science and pseudoscience, falsifiability has featured prominently in many controversies and applications, used as legal precedent.

One concern about the scientific method is how to move from observations to scientific laws. This is the problem of induction. Considering the hypothesis that all swans are white, given an observation of a white swan, there is no logical path from "here is a white swan" to "all swans are white"; doing so would involve a logical fallacy such as, for example, affirming the consequent.