The manipulation and detection of individual quantum excitations forms the basis of modern quantum optics experiments. However, most of these experiments have been restricted to systems composed of only a few particles.
In recent years, tremendous experimental progress has been made in probing strongly interacting many-body systems at the single-particle level. This was achieved by the single-site- and single-atom-resolved imaging and manipulation of quantum gases in optical lattices.
I will review these developments and present, in more detail, a few chosen experiments: The single-site-resolved detection of correlation functions  and the quantum dynamics of a mobile spin impurity . These experiments show that quantum fluctuations of the many-body system become directly visible and that impurities can be implanted and studied with unprecedented control. However, the method has its limits , which I will analyze in the last part of talk. I will also outline future experiments that could overcome some of the restrictions.
 M. Endres, M. Cheneau, T. Fukuhara, C. Weitenberg, P. Schauss, C. Gross, L. Mazza, M. C. Banuls, L. Pollet, I. Bloch and S. Kuhr, Observation of Correlated Particle-Hole Pairs and String Order in Low-Dimensional Mott Insulators, Science 334, 200 (2011)
 T. Fukuhara, A. Kantian, M. Endres, M. Cheneau, P. Schauss, S. Hild, D. Bellem, U. Schollwock, T. Giamarchi, C. Gross, I. Bloch and S. Kuhr, Quantum dynamics of a mobile spin impurity, Nature Phys. 9, 235 (2013)
 M. Endres, M. Cheneau, T. Fukuhara, C. Weitenberg, P. Schauss, C. Gross, L. Mazza, M. C. Banuls, L. Pollet, I. Bloch and S. Kuhr, Single-site- and single-atom-resolved measurement of correlation functions, arXiv:1303.5652 (2013), to appear in Applied Physics B