The function of the centrifugal compressor inducer is to provide wide flow margins from the design flow to the stall and choke flow limits, together with high static pressure recovery. At transonic conditions the inducer becomes the critical to impeller performance in that shock losses and blockage growth diminish stable flow range and may trigger near vertical stage characteristics. The paper covers the various types of inlet configurations upstream of the inducer, followed by a review of the state-of-art inducer design for centrifugal compressors, culminating with some research developments in transonic inducer blading characteristics.