Abstract: To prepare N-phosphonomethyl glycine,a) aminomethylphosphonic acid or one of its salts in water, if necessary or desirable with the addition of lyes, is reacted with an alkali carbonate and/or alkali hydrogencarbonate or with carbon dioxide and a lye,b) the resulting alkali salt of N-phosphonomethylcarbamic acid is subsequently hydroxymethylated with formaldehyde,c) the salts, resulting from b), of N-hydroxymethyl-N-phosphonomethyl-carbamic acid, if necessary or desirable with addition of a lye, are reacted with hydrocyanic acid and/or a cyanide andd) the N-carboxy-N-phosphonomethylglyconitrile salts thus obtained are treated with acids and converted by means of hydrolysis and decarboxylation into N-phosphonomethyl glycine.In this way, it is possible to obtain excellent yields of high-purity N-phosphonomethyl glycine.