We discuss the observational consistency, possible properties, and detection of collapsed nuclei CA. These may be considered as elementary particles with mass number A>1 and of much smaller radius than ordinary nuclei NA. The existence of CA of (perhaps much) lower energy than NA is observationally consistent if NA are very long-lived isomers against collapse because of a "saturation" barrier between CA and NA. Barrier-penetrability estimates show that sufficiently long lifetimes >~1031 sec are plausible for A>~16-40. The properties of CA are discussed using composite baryon and quark models; small charges and hypercharges and, especially, neutral CA are possible. CA can be effectively a source or sink of baryons. Some astrophysical implications are briefly discussed, in particular the possible large scale presence of CA and the possibility that accelerated collapse in massive objects may be a source of energy comparable to the rest mass.