Book excerpt: Helium nanodroplets are the smallest known objects exhibiting superfluidity, and they provide a unique cryogenic matrix for high-resolution spectroscopy and ultracold chemistry applications. The relatively simple electronic structure of helium atoms and the homogeneity of the quantum fluid clusters make them excellent model systems for the study of electronic structures and dynamics in complex systems and the emergence of collective phenomena. Coupled electronic-nuclear dynamics in laser-excited helium nanodroplets are studied in the time-domain in two distinctly different excitation regimes following either single XUV-photon excitation or strong-field ionization by a near-infrared (NIR) laser pulse. Femtosecond time- resolved XUV photoelectron spectroscopy and femtosecond time-resolved X-ray coherent diffractive imaging are employed to monitor electronic relaxation dynamics in neutral droplets and the emergence and evolution of a nanoplasma in strong-field ionized droplets, respectively. Electronically excited pure and doped He droplets are prepared using femtosecond XUV pump pulses produced by high-order harmonic generation. The excited states and subsequent relaxation dynamics are probed by ionization of transient species with a femtosecond UV probe- pulse. Pump-probe time delay-dependent photoelectron kinetic energy distributions are measured using velocity map imaging. In pure droplets excited with 23.7 eV, three dynamic pathways are identified: interband relaxation from the initially excited 1s3p,1s4p manifold to the 1s2p band, further relaxation within the 1s2p Rydberg band and rapid atomic reconfiguration involving formation of Rydberg-excited (Hen)* cores within the droplet. Ongoing efforts towards understanding energy- and charge-transfer mechanisms between the host droplets and dopant atoms are discussed. New high-harmonic generation schemes were implemented in order to directly access the lower 1s2p droplet resonances. Droplets doped with a small amounts of Kr and Ne atoms (ndopant/ndroplet