\gtrsim 10~\mathrm{pc}$, they resemble both the nuclei of low-mass galaxies and the most massive GCs [@brodie2011; @norris2014; @forbes2014; @janz2016]. As they are typically found in the densest environments, environmental effects, such as tidal stripping, are thought to be involved in their formation [@bekki2003; @pfeffer2013].

In a previous paper, we looked at the UDGs and UCDs in Abell 2744 [@janssens2017], the first cluster observed by the Frontier Fields (FF) program with the *Hubble Space Telescope* (*HST*). In this paper, we now investigate the UDGs and UCDs inhabiting all six FF galaxy clusters. In addition to being the most massive and distant systems in which UDGs have yet been discovered, the existing lensing and X-ray analyses permit detailed study of their local environments. This paper is organized as follows. In Section \[sec:data\], we describe the FF program and its data. Our methods, including UDG and UCD selection, are described in Section \[sec:methodology\]. In Section \[sec:results\], we present and discuss the results of our analysis, primarily the abundance of UDGs in the six FF clusters and their spatial distributions in relation to other classes of galaxies and known substructures in the clusters.

We adopt a $\Lambda$CDM cosmology with $\Omega_m = 0.3$, $\Omega_{\Lambda} =
