One-dimensional models are described that can be used for preliminary analysis of the performance of a radial turbocompressor stage. The simple 1D mean-line steady flow analysis given here facilitates a general understanding of the influence of the design parameters on performance and provides a good basis for the preliminary design. The important loss mechanisms and their contribution to the stage efficiency are explained. The physical background to different loss sources is explained, including profile losses, leading-edge losses, trailing-edge losses, end-wall losses, tip clearance losses and leakage losses. An overview of the available correlations is given. A broad outline is given of the state-of-the-art performance levels that can be achieved and the associated design parameters. A meanline calculation procedure for a compressor stage is described for preliminary performance estimation and initial compressor sizing. The effect of parasitic losses on performance is described.