Metal nanoparticles (NPs) have demonstrated to be very versatile for a wide variety of applications that span from localized imaging to targeted drug delivery. The reason why this is possible is based on the possibility to covalently functionalize the metallic core with multiple capping agents taking advantage of the affinity of the metal for thiol- and amino- moieties. The extra stability induced by covalently bound ligands allows one to engineer metal NPs targeting specific applications. In this talk I will describe the functionalization of spherical gold and silver NPs with small molecules, peptides, oligonucleotides and bigger biomolecules, such as antibodies. These NPs have sizes ranging from 1 to 30 nm and interesting optical and electrochemical properties. Steric hindrance, intermolecular hydrogen bonds and thermal properties of these systems will be discussed in detail. Metal NPs with diameters larger than 10 nm are capable of inducing surface enhancement effects and their use in relation to surface enhanced Raman spectroscopy (SERS) will be described. Finally, I will give some examples of the use of these NPs for the development of biosensors and future applications of these systems will be presented.