Abstract: DNA contains the genetic code for all life. The nucleic acid bases that comprise DNA readily absorb ultraviolet light, making them susceptible to photodamage that can lead to mutations and skin cancer. Knowledge of the pathways by which excess electronic energy is dissipated in DNA is important for understanding how photodamage to DNA occurs. The aim of this work is to understand the effect of sequence and secondary structure on the excited-state dynamics in a diverse set of DNA systems. The excited-state dynamics of various GC-containing oligonucleotides were studied by femtosecond transient absorption spectroscopy. Long-lived states with lifetimes at least an order of magnitude slower than the constituent monomers were observed in all the systems studied. Significantly altering the structural conformation of d(GC)9 d(GC)9 through the use of high ionic conditions and low pH revealed that long-lived excited state lifetimes were nearly independent of base stacking geometries and base pairing motifs. The electronic coupling for charge recombination in the long-lived exciplex states of d(GC)9 d(GC)9 is proposed to be large and not significantly altered by helix conformation. The driving force, rather than electronic coupling, for charge recombination is proposed to influence strongly the decay of the long-lived exciplex states in this duplex. Transient absorption experiments were carried out on a genomic DNA sequence to study the effect of sequence disorder on excite-state dynamics. Long-lived excited states were observed in the individual single strands as well as in the duplex of the ras61 oligodeoxynucleotide. The lifetimes of the individual strands were sensitive to composition, with a slower decay observed for the purine-rich strand compared to the pyrimidine-rich strand. In addition, a large ultrafast decay component was observed in the duplex suggesting monomer-like decay pathways are accessible in well-stacked DNA systems. This study shows that long-lived excited states are not an artifact of well-ordered model systems, but are also present in genomic DNA. Isotopic studies revealed a pronounced deuterium isotope effect on the excited-state dynamics of alternating GC-containing oligonucleotides that is not present in non-alternating GC-duplexes. This result parallels those of similar AT-DNAs and is proposed to involve interstrand proton transfer initiated by the formation of an exciplex state with significant charge transfer character. To investigate further the possibility of interstrand proton transfer in DNA duplexes, a series of A- and T-containing oligonucleotides were studied by UV-pump / mid-IR probe spectroscopy. Unique, strong positive bands are observed in alternating AT-DNA, but not in the single- and double-strand non-alternating AT-oligomers. These bands are proposed to be marker bands for proton transfer. Possible absorption bands for excimer states were also observed in (dA)1 in the spectral region of 1500 - 1800 cm−1, supporting previous measurements on polyA.