Electrochemical biosensors based on the use of NPs as electroactive labels offer several advantages in terms of cost efficiency in comparison to traditional methods of bioanalysis such as enzyme-linked immunosorbent assay (ELISA) or polymerase chain reaction (PCR). NPs can be used in a variety of bioanalytical assays with electrochemical detection [7]. When NPs are used as quantitation tags, an electrical/electrochemical signal emanating from the particles is quantified. Encoded NPs used as labels rely on one or more identifiable characteristics to allow them to serve as encoded electrochemical hosts for multiplexed bioassays. This is analogous to the positional encoding of assays on microarrays but in solution. In this context, gold nanoparticles (AuNPs) stand out from the variety of NPs used as labels in biosensing due to their simple synthesis, narrow size distribution, optical and electrochemical properties, and easy bioconjugation alternatives [8]. The advantageous properties of AuNP-based immuno-and deoxyribonucleic acid (DNA) electrochemical assays have given rise to an increased number of publications and other reports in the last years [9]. The aim of this chapter is to present the different strategies for the direct (after NP dissolution or through the NP redox properties) and indirect (electrocatalytic or blocking properties within nanochannels) electrochemical detection of NP tags in immunosensing and DNA hybridization assays. Furthermore different platforms (i.e., magneto-screen-printed electrodes, nanoporous surfaces, etc.) that improve the performance of the developed assays have been used to improve the biosensing performances. The optimized biosensors have been applied for the detection of biomolecules of clinical interest, such as tumor cells or cancer biomarkers in blood. The obtained results show that the developed technologies can be valid alternatives to the traditional methods and are currently the objective of extensive research.