In this paper we study the Martin boundary at infinity for a large class of purely discontinuous Feller processes in metric measure spaces. We show that if ∞ is accessible from an open set D, then there is only one Martin boundary point of D associated with it, and this point is minimal. We also prove the analogous result for finite boundary points. As a consequence, we show that minimal thinness of a set is a local property. ∞ 0 1 (Xt∈A) dt = A G(x, y)m(dy). Moreover, G(x, y) = G(y, x) for all x, y ∈ X, cf. VI.1 in [3] for details. Further, let D be an open subset of X and τ D = inf{t > 0 : X t / ∈ D} the exit time from D. The killed process X D is defined bywhere ∂ is an extra point added to X. The killed process X D is defined analogously. By Hunt's switching identity (Theorem 1.16 for all x, y ∈ X which implies that X D and X D are in duality, see p.43 in [13]. Again by VI.1 in [3], X D admits a unique (possibly infinite) Green function (potential kernel) G D (x, y) such that for every non-negative Borel function f ,the Green function of X D . It is assumed throughout the paper that G D (x, y) = 0 for (x, y) ∈ (D × D) c . We also note that the killed process X D is strongly Feller, see e.g. the first part of the proof of Theorem on pp. 68-69 in [11]. From now on, we will always assume that D is Greenian, that is, the Green function G D (x, y) is finite for all x, y ∈ D, x = y. Under this assumption, the killed process X D is transient in the sense that there exists a non-negative Borel function f on D such that 0 < G D f < ∞ (and the same is true for X).Recall that z ∈ ∂D is said to be regular with respect to X if P z (τ D = 0) = 1 and irregular otherwise. We will denote the set of regular (respectively irregular) points of ∂D with respect to X by D reg (respectively D irr ). D reg (respectively D irr ) stands for the sets of regular (respectively irregular) points of ∂D with respect to X respectively. It is well known that D irr and D irr are semipolar, hence polar under A.The process X, being a Hunt process, admits a Lévy system (J, H) where J(x, dy) is a kernel on the state space X (called the Lévy kernel of X), and H = (H t ) t≥0 is a positive continuous additive functional of X. We assume that H t = t so that for every function f : X × X → [0, ∞) vanishing on the diagonal and every stopping time T ,By using τ D in the displayed formula above and taking f (where ζ is the life time of X. Similar formulae hold for the dual process X and J(x, dy)m(dx) = J(y, dx)m(dy).Assumption C: The Lévy kernels of X and X are of the form J(x, dy) = j(x, y)m(dy), J(x, dy) = j(x, y)m(dy), where j(x, y) = j(y, x) > 0 for all x, y ∈ X, x = y. The next two related assumptions control the decay of the density j. Assumption C1(z 0 , R): Let z 0 ∈ X and R ≤ R 0 . For all 0 < r 1 < r 2 < R, there exists a constant c = c(z 0 , r 2 /r 1 ) > 0 such that for all x ∈ B(z 0 , r 1 ) and all y ∈ X \ B(z 0 , r 2 ),In the next assumption we require that the localization radius R 0 = ∞. Assumption C2(z 0 , R): Let z 0 ∈ X and R > 0. For all...