Anisotropies of the Cosmic Microwave

— We review the present status of Cosmic Microwave Background (CMB) anisotropy observations and discuss the main related astrophysical issues, instrumental effects and data analysis techniques. We summarise the balloon-borne and ground-based experiments that, after COBE-DMR, yielded detection or significant upper limits to CMB fluctuations. A comparison of subsets of combined data indicates that the acoustic features observed today in the angular power spectrum are not dominated by undetected systematics. Pushing the accuracy of CMB anisotropy measurements to their ultimate limits represents one of the best opportunities for cosmology to develop into a precision science in the next decade. We discuss the forthcoming sub-orbital and space programs, as well as future prospects of CMB observations. PACS 98.80 – Cosmology. The Cosmic Microwave Background (CMB) radiation has played a central role in modern cosmology since the time of its discovery by Penzias and Wilson in 1965 [1]. The existence of a background of cold photons was predicted several years before by Gamow, Alpher and Herman [2, 3] following their assumption that primordial abundances were produced during an early phase dominated by thermal radiation. Traditionally, the CMB is considered one of the three observational pillars supporting the cosmological scenario of the Hot Big Bang, together with light elements primordial abundances (see, e.g., [4]) and the expansion of the Universe [5]. In recent years, CMB measurements are widely considered as the single most fruitful field in observational cosmology, thanks to the richness and precision of information it can provide directly from the early universe. In the first decade or so after the discovery, experiments and theoretical work focussed on establishing the nature of the CMB itself, leading to a compelling evidence of a cosmic origin of the radiation. By the early 80's the Hot Big Bang prediction of a highly isotropic background [3] with a nearly Planckian spectrum was remarkably supported by observation. The growing CMB community then shifted the interest to observations of first-order deviations from the " idealised " , unperturbed scenario. Spectrum experiments c Società Italiana di Fisica