  We investigate the generalized tree properties and guessing model properties
introduced by Wei\ss\ and Viale, as well as natural weakenings thereof,
studying the relationships among these properties and between these properties
and other prominent combinatorial principles. We introduce a weakening of Viale
and Wei\ss's Guessing Model Property, which we call the Almost Guessing
Property, and prove that it provides an alternate formulation of the slender
tree property in the same way that the Guessing Model Property provides and
alternate formulation of the ineffable slender tree property. We show that
instances of the Almost Guessing Property have sufficient strength to imply,
for example, failures of square or the nonexistence of weak Kurepa trees. We
show that these instances of the Almsot Guessing Property hold in the Mitchell
model starting from a strongly compact cardinal and prove a number of other
consistency results showing that certain implications between the principles
under consideration are in general not reversible. In the process, we provide a
new answer to a question of Viale by constructing a model in which, for all
regular $\theta \geq \omega_2$, there are stationarily many $\omega_2$-guessing
models $M \in \mathscr{P}_{\omega_2} H(\theta)$ that are not
$\omega_1$-guessing models.
