Abstract. We study numerically and analytically the dynamics of passive energy transfer from a damped linear oscillator to an essentially nonlinear end attachment. This transfer is caused by either fundamental or subharmonic resonance capture, and in some cases is initiated by nonlinear beat phenomena. It is shown that, due to the essential nonlinearity, the end attachment is capable of passively absorbing broadband energy at both high and low frequencies, acting, in essence, as a passive broadband boundary controller. Complicated transitions in the damped dynamics can be interpreted based on the topological structure and bifurcations of the periodic solutions of the underlying undamped system. Moreover, complex resonance capture cascades are numerically encountered when we increase the number of degrees of freedom of the system. The ungrounded essentially nonlinear end attachment discussed in this work can find application in numerous practical settings, including vibration and shock isolation of structures, seismic isolation, flutter suppression, and packaging. 1. Introduction. We study passive and irreversible energy transfer from a linear oscillator to an essentially nonlinear attachment, which, in essence, acts as a nonlinear energy sink (NES); such energy transfer we refer to as nonlinear energy pumping. In previous works (Vakakis and Gendelman (2001), Vakakis et al. (2003)) grounded and relatively heavy nonlinear attachments were considered, a feature that limits their attractiveness in practical applications. To eliminate these restrictions, an ungrounded and light nonlinear attachment is considered in this work, which, in addition, possesses the feature of modularity. As shown in Lee et al. (2005), even though the system considered has a simple configuration, it possesses a very complicated structure of undamped periodic orbits, which, in turn, give rise to a complicated