In this review, we report DNA duplex-based fluorescence probes/sensors using pyrene monomer-excimer switching. The review mainly comprises two topics: 1) excimer-monomer switching molecular beacons (EMS-MB) and 2) monomer-excimer switching sensors based on the structural motif of antibodies. The EMS-MBs have two pyrene fluorophores connected both at 3′ and 5′ ends of a single-stranded oligonucleotide. Emission switching occurs from excimer to monomer accompanying isoemissive points when the probes hybridized with target DNAs. The isoemissive points indicate the presence of only two fluorescent species, nonhybridized and hybridized probes in the mixtures, and thereby unambiguous detection of the targets is available. The probes can detect target 19-mer DNAs and can discriminate the targets from their single-nucleotide mismatches at 1 nM concentration. Furthermore, the EMS-MBs have been recently applied to kinetic study for RNase H activity by Tan et al. The structures and emission-switching properties of the EMS-MBs encourage us to develop a new class of fluorescent sensors based on the structural motif of antibodies. The sensors consist of three functional regions, benzo-15-crown-5 ether (or per-O-methylated β-cyclodextrin), DNA, and pyrene as guest-binding, dimerizing, and sensing sites, respectively. The crown- and CD-modified sensors can detect potassium cation and porphyrin derivatives, respectively, by monomer-excimer emission switching.