T(m) is defined as Temperature of melting or, more accurately, as temperature of midtransition. This term is often used for nucleic acids (DNA and RNA, oligonucleotides and polynucleotides). A thermal denaturation experiment determines the stability of the secondary structure of a DNA or RNA and aids in the choice of the sequences for antisense oligomers or PCR primers. Beyond a simple numerical value (the T(m)), a thermal denaturation experiment, in which the folded fraction of a structure is plotted vs. temperature, yields important thermodynamic information. We present the classic problems encountered during these experiments and try to demonstrate that a number of useful pieces of information can be extracted from these experimental curves.