A narrow system is a combinatorial object introduced by Magidor and Shelah in connection with work on the tree property at successors of singular cardinals. In analogy to the tree property, a cardinal κ satisfies the narrow system property if every narrow system of height κ has a cofinal branch. In this paper, we study connections between the narrow system property, square principles, and forcing axioms. We prove, assuming large cardinals, both that it is consistent that ℵω+1 satisfies the narrow system property and $\square _{\aleph _\omega , < \aleph _\omega } $ holds and that it is consistent that every regular cardinal satisfies the narrow system property. We introduce natural strengthenings of classical square principles and show how they can be used to produce narrow systems with no cofinal branch. Finally, we show that the Proper Forcing Axiom implies that every narrow system of countable width has a cofinal branch but is consistent with the existence of a narrow system of width ω1 with no cofinal branch.

We formulate combinatorial principles that combine the square principle with various strong forms of the diamond principle, and prove that the strongest amongst them holds in L for every infinite cardinal.

As an application, we prove that the following two hold in L:

1. 1. For every infinite regular cardinal λ, there exists a special λ+-Aronszajn tree whose projection is almost Souslin;

2. 2. For every infinite cardinal λ, there exists a respecting λ+-Kurepa tree; Roughly speaking, this means that this λ+-Kurepa tree looks very much like the λ+-Souslin trees that Jensen constructed in L.

If M is a proper class inner model of ZFC and $\omega _2^{\bf{M}} = \omega _2 $, then every sound mouse projecting to ω and not past 0¶ belongs to M. In fact, under the assumption that 0¶ does not belong to M, ${\bf{K}}^{\bf{M}} \parallel \omega _2 $ is universal for all countable mice in V.

Similarly, if M is a proper class inner model of ZFC, δ > ω1 is regular, (δ+)M = δ+ and in V there is no proper class inner model with a Woodin cardinal, then ${\bf{K}}^{\bf{M}} \parallel \delta $ is universal for all mice in V of cardinality less than δ.

We study the consequences of stationary and semi-stationary set reflection. We show that the semi-stationary reflection principle implies the Singular Cardinal Hypothesis, the failure of the weak square principle, etc. We also consider two cardinal tree properties introduced recently by Weiss, and prove that they follow from stationary and semi-stationary set reflection augmented with a weak form of Martin’s Axiom. We also show that there are some differences between the two reflection principles, which suggests that stationary set reflection is analogous to supercompactness, whereas semi-stationary set reflection is analogous to strong compactness.