The time scale for the Cambrian through Early Devonian periods is revised using new U–Pb ages from volcanic and pyroclastic rocks in sequences that are stratigraphically well defined. A rhyolite flow from the latest Precambrian, dated at 551.4 ± 5.8 Ma, gives a maximum age for the base of the Cambrian (Phycodes pedum Zone) at Fortune Bay, Newfoundland. The age ranges of several Ordovician and Silurian series are now established to within 2–3 Ma based on stratigraphically controlled samples from Britain. A basal Llandeilo ash has an age of 460.4 ± 2.2 Ma; a basal Caradoc lava has an age of 456.1 ± 1.8 ± Ma; and K-bentonites from the latest Llandovery and latest Ludlow have ages of 430.1 ± 2.4 and 420.2 ± 3.9 Ma, respectively. Using other data from the literature, we interpolate the base of the Cambrian to be near 545 Ma, the base of the Ordovician as ca. 495 Ma, and the base of the Silurian as ca. 443 Ma. While we have a good estimate for the base of the Middle Devonian (at ca. 391 Ma), the base of the Devonian is less precise, probably around 417 Ma.