This chapter covers the most relevant literature published during the year 2018, related to tervalent phosphorus acid derivatives. Some of the most relevant concepts are reported, regarding synthetic methodologies to halogenophosphorus compounds, phosphorus amides (aminophosphines, phosphoramidites and diamidophosphites), phosphorus esters (phosphinites, phosphonites and phosphites) and mixed phosphorus compounds bearing two different tervalent phosphorus moieties. A critical selection of their applications is briefly discussed, with an emphasis on the catalytic applications of ligands and metal complexes.