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In this review I briefly discuss the theory of pre-main-sequence (PMS) Li depletion in low-mass (0.075 < M < 1.2 M⊙) stars and highlight those uncertain parameters which lead to substantial differences in model predictions. I then summarise observations of PMS stars in very young open clusters, clusters that have just reached the ZAMS and briefly highlight recent developments in the observation of Li in very low-mass PMS stars. [ABSTRACT FROM AUTHOR]

We investigate the issues of contamination and exclusion in photometric membership selection techniques when applied to OB associations. We use radial velocities to calculate membership probabilities for a sample of very low-mass candidate members of the σ Orionis young group. We find that significant numbers of bona members are not excluded by photometric selection techniques, and for I brighter than 17 there is not significant contamination in the expected PMS region. [ABSTRACT FROM AUTHOR]