Astronomie Spatiale Infrarouge, Aujourd'hui Et Demain Infrared Space Astronomy, Today and Tomorrow : Springer : 9783540673866 : 3540673865 : 19 Jul 2000 : In these lectures, I have discussed a number of basic concepts that provide the necessary background to the current studies of star formation. A ?rst partwas dedicatedto illustrate the conceptofa protostar,discussing con- tions and propertiesof the collapseof a molecular core. A secondpart deals with circumstellardisks. Disks areimportantnot only to the processofstar formation itself, but also because they are in all probability the site where planets form. The age range of pre-main?sequence stars coincides with the timescales for the formation of very large planetesimals, the building blocks of planets. Studies ofdisk properties in pre-main?sequencestars ofdi?erent age, located in star-forming regions of di?erent properties, may shed light on the characteristics of planet formation processes. ISO observations can provide important (