In this paper, I attempt to define the semantics of cikum ``now`` in Korean. To define it precisely, we need to look at how it interacts with different aspectual classes of verbs and with the semantics of tenses and aspects. In doing this we need to define the semantics of tenses and aspects. Here we run into the question of whether ess is a tense marker or an aspect marker. I assume that it is ambiguous. There are still cases where it is not clear whether ess is used as a tense marker or an aspect marker in an actual sentence. I discuss two such cases: one in which it is used with verbs like tochakha ``arrive`` which have no salient resulting states, and one in which a state verb is used with cikum-kkaci ``until now``. The semantics of cikum can be defined differently depending on whether ess is a tense or an aspect. By discussing ko iss, which is an imperfect marker, I conclude that cikum means λPλi[u ⊆ i ∧ P(i)], that is, a relation between a set of times which include an utterance time and a set of properties of times.