The combination of efficient photoluminescence (PL) and ease of processing has generated much interest in conjugated polymers with respect to their use in light emitting diodes (LEDs) and other optoelectronic devices [1][2][3][4][5]. This interest has, in turn, led to extensive investigations into the nature and dynamics of photoexcited states. Photoexcitation dynamics in conjugated polymers span a wide range of timescales. Singlet excitons self-trap and migrate on a subpicosecond timescale and decay within approximately one nanosecond. Long-lived photoexcitations such as polarons, bipolarons, and triplet excitons are formed by the non-radiative decay of singlet excitons and have lifetimes exceeding a millisecond at low temperatures. As a consequence, multiple spectroscopic techniques are required to understand the photophysics of these novel materials.We have investigated the photophysics of conjugated polymers using a variety of linear and non-linear spectroscopies. Photomodulation spectroscopy (PM) uses two light sources whereby absorption of a pump beam modulates the transmission or reflectance of a probe beam incident on the sample. PM spectroscopy can detect a variety of phenomena, depending upon the timescale of the measurement. Transient PM studies with nanosecond or better resolution have detected stimulated emission (SE) and photoinduced absorption (PA), also known as excited state absorption. Continuous wave (CW) PM spectra include contributions from PA, photoluminescence (PL) from spontaneous emission, and electroabsorption. The latter signal is caused by electric fields arising from photogenerated charged excitations. As photoexcitation dynamics in conjugated polymers span a wide range of timescales, we have employed both transient and CW PM spectroscopies to detect photoexcitations and measure their relaxation dynamics. Optically detected magnetic resonance (ODMR) has also been employed to determine the spin of long-lived photoexcitations.Conjugated polymers are centrosymmetric systems where excited states have definite parity of even (A g ) or odd (B u ) and electric dipole transitions are allowed only between states of opposite parity. The ground state of conjugated polymers is an even parity singlet state, written as the 1A g . PM spectroscopy is a linear technique probing dipole allowed one-photon transitions. Non-linear spectroscopies complement these measurements as they can couple to dipole-forbidden Semiconducting Polymers: Chemistry, Physics and Engineering. Edited by G. Hadziioannou and P. F. van Hutten