The Planck low frequency instrument

The Low Frequency Instrument (LFI) of the "Planck Surveyor" ESA mission wil l perform high-resolution imaging of the Cosmic Microwave Background anisot ropies at four frequencies in the 30-100 GHz range. We review the LFI main scientific objectives, the current status of the instrument design and the on-going effort to develop software simulations of the LFI observations. In particular we discuss the design status of the PLANCK telescope, which is critical for reaching adequate effective angular resolution.