Arthropods are the most diverse animal phylum, and their phylogenetic relationships have been debated for centuries. With the advent of molecular phylogenetics, arthropods were found to be monophyletic and placed within a clade of molting animals, the ecdysozoans, with nematodes and six other phyla. Molecular phylogenetics also provided a new framework for relationships between the major arthropod groups, such as the clade Pancrustacea, which comprises insects and crustaceans. Phylogenomics based on second-generation genomics and transcriptomics has further resolved puzzles such as the exact position of myriapods or the closest crustacean relatives of hexapods. It is now broadly recognized that extant arthropods are split into chelicerates and mandibulates, and relationships within the two mandibulate clades (myriapods and pancrustaceans) are stabilizing. Notably, the phylogeny of insects is now understood with considerable confidence, whereas relationships among chelicerate orders remain poorly resolved. The evolutionary history of arthropods is illuminated by a rich record of fossils, often with exquisite preservation, but current analyses conflict over whether certain fossil groups are stem- or crown-group arthropods. Molecular time-trees calibrated with fossils estimate the origins of arthropods to be in the Ediacaran, while most other deep nodes date to the Cambrian. The earliest stem-group arthropods were lobopodians, worm-like animals with annulated appendages. Confidently placing some key extinct clades on the arthropod tree of life may require less ambiguous interpretation of fossil structures and better integration of morphological data into the phylogeny.