Intercst III tllin films arIses both from a theoreticalrnpoint of view and from the wide range of applications theyrncan offer. All the advantages and applications of thi.nrnfil ms a 1'e too many to men tion, But some of them are, theirrnuse, as compact technological devices, corrosion protections,rnoptical instrulIlcnts and sensors of scientific research.rnThe aim of the present paper consists in the theoreticalrninvestiga tion of firstÂ·~order phase transitions in physi ..rnsOTbed monolayers. Hereby, mainly thermodynamic methods arernapplied. Therefore, first a thermodynamic theory of homogeneousrnaJld heterogeneous monolayers is developed. In parti ..rncular, general ('quations for the curvature dependence of thernJ ine tension are obtained. It is shown tha t cm'va turerndependence of 1. ine tension is of importance only for veryrnsmall 2-d clusters. A general thermodynamic analysis ofrnfirst-6rder phase transitiollS, starting from metastablernin:itial states, is given. It is shown that in real situations,rnwilen depl etion effec ts are ta ken into accoun t, the trans i tionrnproceeds via three main stages: a stage of nucleation andrnpossible simultolleous growth, n stage of independent growthrnof the clusters, their number being nearly constant and arnthird stage of cOlllpeti tive grO'lth of Ostwald ripening. Thern'j'esults are analogous to phase transitions in 3-d systems, asrnwas discussed by Schmelzer [19, 20], Ulbrich eLal [1]. Basedrnon the thermodynamic investiga tiOll, a kinetic desc ription ofrnthe growth of single and ensemble of 2-d clusters is developed.rnIt is shown tha t in the s ta ge of independen t growth, thernmean radius of the ensemble of clusters grollls as, R" t forrnkinetic limited growth and as R2.;, t for diffusion limitedrngrOlvth. Moreover, a kinetic theory of Ostwald ripening inrnmonolayer:> is developed which allol'ls i1 clesc ription of the who 1 erncourse of the process 0 In the asymptotic region, the result5rncoincide with the predictions obtained first by Lifshit.z andrn51 yo zov [23].rnThe resu1t5 are used for a theoretical explilllct 1.ion of therncomputer simulation of a first-order phase transition III arnphysisorbed monolayer carried out by Kock eLaI. [4] 0 Thatrnis, first the asymptotic resul t of R2", t for the case of Ostwaldrnripening in kinetic-"l imited growth is reconfirmed. Inrnaddition, the developed theory allows the interpretation forrnthe initial slo'1 increase of the mean cluster size, R, in thernfirst stage of Ostwald ripening.rnThe analysis, carried out here, is for isothermal conditionsrnand one component systerns. A general iza tion to otherrntypes of constraints, e.g" adiabatic conditions and multicomponentrnsystems is possible