This chapter focuses on the spectral analysis and long-time asymptotics of several classes of structured population models. We develop a spectral gap theory for a class of integro-differential models of diffusive type, subject to generalized Wentzell-Robin boundary conditions. We also address growth-fragmentation equations (conservative or with mass loss) in different \(L^{1}\) -settings, under various assumptions on the growth and fragmentation rates; this leads to several theories, including one describing a scattering regime (runaway phenomenon). Additionally, we explore growth equations without fragmentation and with a nonlocal McKendrick-von Foerster boundary condition.