Phenomenology is our way of access to what is to be the theme of ontology, and it is our way of giving it demonstrative precision. Only as phenomenology, is ontology possible. In the phenomenological conception of "phenomenon" what one has in mind as that which shows itself is the Being of entities, its meaning, its modifications and derivatives.1 And this showing-itself is not just any showing-itself, nor is it some such thing as  appearing. Least of all can the Being of entities ever be anything such that 'behind it' stands something else 'which does not appear'.
'Behind' the phenomena of phenomenology there is essentially nothing else; on the other hand, what is to become a phenomenon can be hidden. And just because the phenomena are proximally and for the most part not given, there is need for phenomenology. Covered-up-ness is the counter-concept to 'phenomenon'.
There are various ways in which phenomena can be covered up. In the first place, a phenomenon can be covered up in the sense that it is still quite undiscovered. It is neither known nor unknown.2 Moreover, a phenomenon can be buried over [verschüttet]. This means that it has at some time been discovered but has deteriorated [verfiel] to the point of getting covered up again. This covering-up can become complete; or rather—and as a rule—what has been discovered earlier may still be visible, though only as a semblance. Yet so much semblance, so much 'Being'.3 This covering-up as a 'disguising' is both the most frequent and the most dangerous, for here the possibilities of deceiving and misleading are especially stubborn. Within a 'system', perhaps, those structures of Being—and their concepts—which are still available but veiled in their indigenous character, may claim their rights. For when they have been bound together constructively in a system, they present themselves as something 'clear', requiring no further justification, and thus can serve as the point of departure for a process of deduction.
The covering-up itself, whether in the sense of hiddenness, burying-over, or disguise, has in turn two possibilities. There are coverings-up which are accidental; there are also some which are necessary, grounded in what the thing discovered consists in [der Bestandart des Entdeckten]. Whenever a phenomenological concept is drawn from primordial sources, there is a possibility that it may degenerate if communicated in the form of an assertion.
1 'Der phänomenologische Begriff von Phänomen meint als das Sichzeigende das Sein des Seienden, seinen Sinn, seine Modifikationen und Derivate.'
2 'Über seinen Bestand gibt es weder Kenntnis noch Unkenntnis. The earlier editions have 'Erkenntnis' where the latter ones have 'Unkenntnis'. The word 'Bestand' always presents difficulties in Heidegger; here it permits either of two interpretations, which we have deliberately steered between: 'Whether there is any such thing, is neither known nor unknown', and 'What it comprises is something of which we have neither knowledge nor ignorance.'
3 'Wieviel Schein jedoch, soviel "Sein".'