The spindle checkpoint ensures accurate chromosome segregation by delaying anaphase onset until all kinetochores have properly established bipolar attachment to spindle microtubules. This mechanism is important for all eukaryotic cells and is evolutionarily conserved. Much of our understanding of the molecular and biochemical mechanisms of the spindle checkpoint has been gained from parallel studies in various experimental systems. In particular, the cytoplasmic extract from the eggs of Xenopus laevis provides an unsurpassable system for biochemical analysis of the spindle checkpoint and has made important contributions to the field. This article reviews the progress of the spindle checkpoint studies in Xenopus laevis with a focus on the regulation by phosphorylation.