This paper describes an experience with a Software Process Improvement (SPI) approach particularly adapted to small structures with low software maturity level (e.g. small and medium enterprises in software businesses, small software teams in bigger businesses, small software teams in public organisations). A characterisation of Software Process in-the-small is first made on basis of a deep analysis of software processes actually followed by a number of small and medium enterprises (SMEs). The approach proposes a gradual Software Process Improvement framework based on three nested models. It allows SMEs to start SPI in a very targeted manner, to quickly progress within a limited budget and, eventually, to reach an acceptable level according to SPI standard models such as CMM and SPICE.