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 (in some cases, unique) inf- mation on the various stages of the star and planet formation. I have illustrated in detail some examples, when, to my knowledge, ISO data had been reduced and analyzed. Many other programs exist, and will certainly contribute to our understanding of star formation in the near future.