Linguistic units such as syllables, words, phrases, and sentences simultaneously unravel during speech processing. How does the brain parse, track, and process these concurrent linguistic units? In this talk, I will discuss our recent efforts to understand whether and how natural speech is processed online, and what role brain rhythms might play in this process. Specifically, I will discuss studies demonstrating that rhythmic cortical activity entrains to the time course of large linguist units, even in the absence of any acoustic cues for the boundaries between phrases and sentences. Similar entrainment is also observed during second language acquisition, and can rapidly appear when learning to parse an artificial language. Together, these studies show that cortical entrainment to linguistic units reliably tracks online speech processing, offering endless possibilities to objectively assess language processing in children, difficult-to-test-populations (e.g., minimally conscious patients), as well as language precursors in animal preparations to allow for cross-species comparison.