5.1 Introduction

Stellar streams represent the remnants of ancient accretion events into a galaxy, and are thus extremely important as tracers of the galaxy formation process. In recent years, it has been increasingly recognized that many of the clues to the fundamental problem of galaxy formation are preserved in fossil substructures (Freeman and Bland-Hawthorn, 2002), particularly in the outskirts of galaxies. Hierarchical formation models suggest that galaxy outskirts form by accretion of minor satellites, predominantly at early epochs when large disk galaxies were assembling for the first time. The size, metallicity, and amount of substructure in the faint outskirts of presentday galaxies are therefore directly related to issues such as the small-scale properties of the primordial power spectrum of density fluctuations and the suppression of star formation in small halos (Springel et al., 2005).

Remarkable progress has been made in recent years in understanding galaxy formation and evolution. High redshift observations have revealed the star formation history of the universe and the evolution of galaxy morphology (see, e.g., Bell et al., 2005; Ryan et al., 2008). However, the nature of look-back observations does not allow one to study the evolution of individual galaxies, and low-mass or small-scale structures remain out of reach in all but the nearest galaxies. Hence high spatial-resolution observations in nearby galaxies are required to complement the samples at cosmological distances to answer many of the big fundamental questions of galaxy formation such as how did the Milky Way build up, and how typical was this formation history?