Many plants use day length as an environmental cue to ensure proper timing of the switch from vegetative to reproductive growth. Day-length sensing involves an interaction between the relative length of day and night, and endogenous rhythms that are controlled by the plant circadian clock. Thus, plants with defects in circadian regulation cannot properly regulate the timing of the floral transition. Here we describe the gene EARLY FLOWERING 4 (ELF4), which is involved in photoperiod perception and circadian regulation. ELF4 promotes clock accuracy and is required for sustained rhythms in the absence of daily light/dark cycles. elf4 mutants show attenuated expression of CIRCADIAN CLOCK ASSOCIATED 1 (CCA1), a gene that is thought to function as a central oscillator component. In addition, elf4 plants transiently show output rhythms with highly variable period lengths before becoming arrhythmic. Mutations in elf4 result in early flowering in non-inductive photoperiods, which is probably caused by elevated amounts of CONSTANS (CO), a gene that promotes floral induction.