Despite recent progress, the first stages of cloud fragmentation andcore collapse remain poorly known. Significant observational advanceswere achieved in this field over the last decade, thanks to the use oflarge (sub)millimeter radiotelescopes such as the IRAM 30 m and theJCMT. Young protostars ("Class 0" objects), featuring powerful jetsand marked infall motions, were identified at the beginning of themain accretion phase. The density structure of numerous prestellarcondensations was also measured, setting strong constraints on theinitial conditions for individual collapse in molecular clouds. Morerecently, the advent of large-format bolometer arrays has allowedcomplete imaging of several nearby protoclusters in thesubmillimeter. The results suggest that stars are generally built fromfinite reservoirs of mass and imply that the stellar initial massfunction (IMF) is at least partly determined by turbulentfragmentation at the prestellar stage of star formation. This fieldwill greatly benefit from future large submillimeter projects such asFIRST/Herschel and ALMA in which Europe is involved at the highestlevel.

We give an overview of the search and discovery ofSequence (PMS)stars in the Magellanic Clouds. For the first time theirregular optical brightness variability of Pre-Main Sequence stars,generally attributed to variable dust obscuration of the central star,has been used as an initial selection of young stellar objects.We discuss 21 PMS candidates in the Large Magellanic Cloud (LMC)and 7 PMS candidates in the Small Magellanic Cloud (SMC) detectedin this way. These stars have bolometric luminosities whichcorrespond to the most massive Galactic Herbig AeBe (HAeBe) stars, or evenmore massive.The location of the LMC and SMC objects is correlated withhigher densities of cold interstellar dust as measured by IRAS inthe far infrared. We derive the fundamental parameters fromspectroscopy and find that there location in the HR-diagram would indicatethat the stars are more luminous when compared to Galactic HAeBe stars ofthe same spectral type. There may be a trend with metallicity, in that theSMC PMS stars can be even more luminous than the LMC PMS stars. Itindicates a possible increase of the proto-stellar accretion rate withdecreasing metallicity.In the optical these HAeBe candidates have a higher probabilityto be found in small (10 pc) clusters then other field stars of similarcolour and magnitude. We present a case study of a cluster aroundone of the LMC HAeBe stars, using high resolution SUSI imaging. TheHα-emitters detected in this cluster are also located in anHR-diagram above the Galactic Palla-Stahler birthline corresponding to anaverage protostellar mass accretion rate of 10-5 M⊙ yr-1.

We review the properties of stellar populations and the interstellarmedium (ISM) of various kinds of massive star forming regions(ranging from stellar clusters, Hii regions, super star clusters,etc. to distant starburst galaxies). The nature of their multi-wavelength spectra as well as the techniques used for their interpretation is are discussed.These objects play an important role in our understandingof star formation, interactions between stars and the ISM, and galaxyevolution among others.

I review the recent instrumental progresses which allow to directlyobserve galaxy properties at the earliest epochs of the Universe.This leads to the derivation of fundamental quantities, such as theepoch of massive galaxy formation or the universal history of heavyelement formation. Star formation history shows a gradual declinesince the last 8-9 Gyr. The bulk of present-day stellar mass and metalcontent was formed at redshifts 0.5 to ~3, which is consistentwith hierarchical scenarii of galaxy formation. The numerousuncertainties related to these determinations are emphasized,including those linked to stellar physics.

The association between star-forming regions and X-rayemission wasdiscovered as soon as X-ray satellites became available, over 30 yearsago. However, understanding of the nature of this association reallystarted when X-ray images and the resulting source identificationcould be obtained. We now know that essentially all young stellarobjects, T Tauri stars and protostars, are X-ray emitters, althoughthe case of the youngest, Class 0 protostars, is less clear. Thepaper briefly reviews X-ray emission and absorption mechanisms, andemphasizes the "solar paradigm" for the interpretation of the X-rayemission, originating in the magnetic activity of young stellarobjects. The possibility of star-disk magnetic interactions inprotostars is also examined. In addition, feedback effects on thecircumstellar disks are presented: ionization, and the associatedcoupling with magnetic fields; particle irradiation, and itsconsequences for the origin of "extinct radioactivities" inmeteorites.

The main properties of pre-main-sequence (PMS) evolution are reviewed, withparticular emphasis on the impact of star formation studies. I will firstdiscuss the basic results established by the work of Hayashi, Iben andCameron in the early '60s. The changes introduced by the constraintsset by the previously ignored phase of protostellar evolution will bethen addressed. Evolutionary models including a variety of physical effects(accretion, rotation, magnetic field) are presented. Finally, theapplication of PMS models to reconstruct the star formation historiesof clusters and associations will be addressed.

This lecture reviews current observational constraints on theoretical models of accretion-driven ejection in young stars. Fundamental properties of jets (collimation scale, kinematics, variability, density, composition) and ejection/accretion ratios are presented and compared at various stages of star formation (Class 0, I, II). Mass-loss rate derivations and evidence for a broader wind component around the jet are also critically examined.

Accretion disks are pivotal elements in the formation and earlyevolution of solar-like stars. On top of supplying the raw material,their internal conditions also regulate the formation of planets.Their study therefore holds the key to solve this long standingmystery: how did our Solar System form? This chapter focuses onobservational studies of the circumstellar environment, and inparticular of circumstellar disks, associated with pre-main sequencesolar-like stars. The direct measurement of disk parameters poses anobvious challenge: at the distance of the typical star forming regions( e.g. ~140 pc for Taurus), a planetary system like ours (withdiameter ≅50 AU out to Pluto, but excluding the Kuiper belt which could extend much farther out) subtends only 0.35''. Yetits surface brightness is low in comparison to the bright central starand high angular and high contrast imaging techniques are required ifone hopes to resolve and measure these protoplanetary disks. Fortunately, capable instruments providing 0.1'' resolution or betterand high contrast have been available for just about 10 yearsnow. They are covering a large part of the electromagnetic spectrum,from the UV/Optical with HST and the near-infrared from ground-basedadaptive optics systems, to the millimetric range with long-baselineradio interferometers. It is therefore not surprising that ourknowledge of the structure of the disks surrounding low-mass stars hasmade a gigantic leap forward in the last decade. In the following pages we will attempt to describe, in a historicalperpective, the road that led to the idea that most solar-like starsare surrounded by an accretion disk at one point in their early lifeand how, nowadays, their structural and physical parameters can beestimated from direct observations. We will follow by a shortdiscussion of a few of the constraints available regarding theevolution and dissipation of these disks. This last topic isparticularly relevant today to understand the mechanism leading to theformation of planets.

In low-mass disks, turbulent torques are probably the most importantway of redistributing angular momentum. Here we present the theory ofturbulent accretion disks. We show the molecular viscosity is far toosmall to account for the evolutionary timescale of disks, and wedescribe how turbulence may result in enhanced transport of (angular)momentum. We then turn to the magnetorotational instability, whichthus far is the only mechanism that has been shown to initiate andsustain turbulence in disks. Finally, we present both the basis andthe structure of α disk models.

In this lecture I review the theory of magnetized accretion discs driving jets, with a focus on Young Stellar Objects (YSOs). I first introduce observational and theoretical arguments in favor ofthe "disc wind" paradigm. There, accretion and ejection are interdependent, requiring therefore to revisit the standard picture of accretion discs. A simple magnetostatic approach is shown to already provide some insights into the basic phenomena. The magnetohydrodynamic equations as well as all usual assumptions are then clearly listed. The relevant physical mechanisms of steady-state accretion and ejection fromdiscs are explained in a model independent way. The results of self-similar calculations are shown and criticallydiscussed, for both cold and warm jet configurations. I finally provide observational predictions and the physical conditions required in YSOs discs. The need for introducing a magnetospheric interaction between the disc and the protostar is briefly discussed.