The advent of multiparameter technology has been driven by the need to understand the complexity in biological systems. It has spawned two main branches, one in the arena of high-content measurements, primarily in microscopy and flow cytometry where it has become commonplace to analyze multiple fluorescence signatures arising from multiple excitation sources and multiple emission wavelengths. Microscopy is augmented by topographical content that identifies the source location of the signature. The other branch involves multiplex technology. Here, the intent is to measure multiple analytes simultaneously. A key feature of multiplexing is an address system for the individual analytes. In planar arrays the address system is spatial, in which affinity reactions occur at defined locations. In suspension arrays, the address is encoded as a fluorescent signature in the particle assigned to a specific reaction or analyte. Several hybrid systems have also been developed for multiplexing. In the commercial regime, the most widespread applications of multiplexing are currently in the areas of genome and biomarker analysis. Planar chips with fixed arrays are now available to probe the entire genome at the level of message expression and large segments of the genome at the level of single nucleotide polymorphism (SNP). In contrast, suspension arrays provide the potential for probing segments of the genome in a customized way, using capture tags that locate specific oligonucleotide sequences to specific array elements.