Synapses are asymmetric intercellular junctions connected by multiple synaptic cell adhesion molecules (CAMs). Synaptic CAMs function in various stages of synaptogenesis - the process of synapse creation - encompassing synapse formation, maturation, refinement, plasticity, and elimination. The list of synaptic CAMs has rapidly grown, although their precise functions of most CAMs at synapses remain incomplete. Members of an emerging class of transmembrane proteins containing leucine-rich repeat (LRR) domains have received considerable recent research attention. In this minireview, I discuss recent findings on LRR-containing synaptic CAMs that impact synapse development and circuit formation, focusing on two families of LRR synaptic CAMs: leucine-rich transmembrane proteins (LRRTMs) and Slit and Trk-like family (Slitrks). Their basic biochemical properties, proposed functions at synapses, physiological significances, and open questions are summarized.